Small stuff (#8)

* cave pearls and spindleshrooms (name subject to change) now are things. Not in mapgen yet.

* add the wandering "gas wisp" to light up some of the gas-filled caverns

* make wisps rarely spawned by gas explosions

* revamp spindlestems into a sort of mineral detector, add glowing extract bottles

* optimize pngs

* add gas wisps to mapgen

* add spindlestems to cavern level 1, most level 1 warrens are now lit up

* update internal names, adjust mineral detection range

* add cave pearls to some level 2 warrens and tunnels

* switch experimental simplecrafting_lib support to crafting mod

* Pearls don't grow on falling nodes

* put spindlestems with goblin caps, make them always grow red when near those

* bunch of documentation

* add castle coral to replace cave coral, which has been repurposed into column decoration

* documentation for cave coral, update some locale text

* add a recipe for cooking oil into paraffin

* add old bones to the underworld

* MIT license for bones_loot

* also cook black cap gills into paraffin, they're oily

* add salt crystals to the bloodthorn caverns, illuminating the floor

* documentation for salt crystals

* auto-generate minetestmapper colors.

need to update the spindlestem colours manually

* add spindlestem to fungiwood caverns too, and increase warren coverage

* in anticipation of eventually adding stuff below the Slade, making glowing pit erosion self-limiting.

* add a bit of displacement to the underside of the slade layer

* Unique images and names for cooking recipes.

* revamp bones loot

* add softer footsteps for some fungus types

* update mapgen_helper

* update cave coral screenshot

* mention glowing salts in bloodthorn caverns

guide.md

@@ -78,6 +78,16 @@ Fungiwoods have thinner stalks that sprout numerous thin irregular "shelves" bra
 
 Plump helmets and cave wheat grow here.
 
+## Spindlestems
+
+![Spindlestems](./df_caverns/screenshots/spindlestems.jpg)
+
+Spindlestem mushrooms are the weedy "saplings" of the upper fungal forests, too large to pick by hand but not exactly inspiring tree-like analogies. They are common, though, and not without their uses. Their long stems can serve as a wood substutitue in many crafts and their bioluminescent caps can produce a long-lived glowing extract that's useful as a source of light. They grow taller when crowded.
+
+The bioluminescence of Spindlestems are actually a very interesting feature. Despite their apparent visual diversity, Spindlestems are a single species. They pick up their glow from symbiotic microorganisms that colonize their caps, and the specific breed of symbiote that thrives in any given Spindlestem depends on the minerals present in the nearby rocks. Iron results in a weak red-glowing cap, copper results in a green-tinted cap, and the combination of both can result in a brighter cyan glow. Brilliant yellow "Golden Spindlestems" have also been spotted.
+
+For some reason, Spindlestems that grow in the presence of Tower Caps are never luminescent. Perhaps they're intimidated.
+
 # Second cavern layer
 
 By default, the second cavern layer can be found between -800 and -1400 meters elevation. Like the first layer, some caverns contain lakes filling their lower regions. The dry barren caverns at this level will have some occasional ruby crystal growths in them, but they will be detailed in an entry on the third cavern layer where they're much more common.
@@ -86,7 +96,9 @@ By default, the second cavern layer can be found between -800 and -1400 meters e
 
 ![Goblin Caps](./df_caverns/screenshots/goblin_cap.jpg)
 
-Goblin caps are the broader, squatter cousins of the Tower Cap fungus. Like Tower Caps they have no particular special properties, but their wood comes in two different colours - the reddish orange of their caps and the cream coloured stem wood. Plump helmets grow here.
+Goblin caps are the broader, squatter cousins of the Tower Cap fungus. Like Tower Caps they have no particular special properties, but their wood comes in two different colours - the reddish orange of their caps and the cream coloured stem wood. Plump helmets and Spindlestems grow here.
+
+For some reason, Spindlestems that grow in the presence of Goblin Caps are always red.
 
 ## Spore Tree caverns
 
@@ -104,6 +116,12 @@ Tunnel tubes are hollow, curved fungal growths that support a fruiting body at t
 
 Sweet pods and cave wheat can be found growing in tunnel tube caverns.
 
+## Cave pearls
+
+![Cave Pearls](./df_caverns/screenshots/cave_pearls.jpg)
+
+At these depths you may find a distinctive and beautiful formation encrusting the ceilings and walls of smaller tunnels and warrens; "cave pearls." These nodules are actually calcified bacterial colonies that can only form where they're left undisturbed for the long periods needed for them to grow this large.
+
 # Third cavern layer
 
 The third cavern layer is where things start to get really exotic. Black Caps, Nethercaps, Bloodthorn and reddish glowing crystal formations are found here. Those caverns that are partly-flooded have unusual features thanks to the strange life sharing them. By default the third cavern layer runs from -1400 to -2000 meters.
@@ -132,7 +150,9 @@ Ice sprites, mysterious glowing insect-like creatures that appear to be made par
 
 Bloodthorns are cactus-like in appearance and habitat, growing in dry sandy caverns, but they are far more agressive about seeking moisture than their surface kin. Bloodthorns have thick spines projecting randomly from their trunks that are coated in smaller barbs that greedily drain fluids from any creature that touches them. You can climb a bloodthorn quite easily, using its spines like rungs on a ladder, but it will hurt to do so. Bloodthorn caverns are dimly lit so take care not to blunder into the rows of bloodthorns that grow like living fences.
 
-Lakes are never found in Bloodthorn caverns. Quarry bushes can be found here.
+Lakes are never found in Bloodthorn caverns and the cracks in the rock that would normally seep moisture are choked with crystals of salt. Some of the salts seem to containe traces of Mese, giving them a faint unearthly glow.
+
+Quarry bushes can be found here.
 
 ## Barren crystal caverns
 
@@ -164,6 +184,8 @@ Venturing below the Sunless Sea will require either a great deal of digging thro
 
 Eventually, at around -2600 meters elevation, an explorer may encounter lakes of oil in lenticular caverns. Falling into a lake of oil is an unpleasant experience - one is completely blind when below the surface - but that's not the real danger. The atmosphere above the oil is filled with unbreathable gasses that are explosive when mixed with air and an ignition source (such as a carelessly placed torch). Even in the tunnels above the oil there are occasional gas seeps venting this gas from cracks and seams of coal. This gas is heavier than air and will pool in depressions, and of course it cannot be swum through so take care not to jump down into a place you can't easily climb back out of.
 
+A few mysterious blue flames may occasionally be found flickering on the edges of oil lakes, burning without oxygen and seeming to maintain their size as they drift about through the unbreathable atmosphere. These "gas wisps" may be a strange form of plasmoidal life but they are easily snuffed and so little is known about them.
+
 Buckets of oil serve as an excellent fuel source, powering a furnace as well as a block of coal.
 
 # Magma Sea
@@ -182,6 +204,8 @@ The foundations of the world lie at -3100 meters, under a default configuration.
 
 There is no native life in the Underworld. It appears there once _was_, however. Occasional clusters of vacant buildings can be found, impossibly crafted from bricks of Slade and empty of any furnishings. Around those clusters of vacant buildings are fields filled with sealed pits lined with flawless Slade blocks. It is unclear whether these pits are as vacant as the buildings, though. The seals capping them are engraved with ancient words in lost tongues, but one phrase can be translated from the oldest known languages: "This Place is Not a Place of Honor."
 
+Just as ominously, the ancient bones of long-dead warriors are scattered amidst the buildings. They contain loot for those willing to risk disturbing them. There are no traces of who - or what - they died fighting.
+
 ![Glowing pit](./df_caverns/screenshots/glowing_pit.jpg)
 
 Also scattered around the Underworld are the occasional twisting shaft leading down into the Slade. The edges of these shafts are encrusted with glowing purple crystals and the bottoms of these shafts are filled with some form of shimmering liquid that destroys whatever tries to penetrate it.