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

df_mapitems/crystals_salt.lua

@@ -0,0 +1,33 @@
+-- internationalization boilerplate
+local MP = minetest.get_modpath(minetest.get_current_modname())
+local S, NS = dofile(MP.."/intllib.lua")
+
+minetest.register_node("df_mapitems:salt_crystal", {
+	description = S("Luminous Salt Crystal"),
+	_doc_items_longdesc = df_mapitems.doc.salt_desc,
+	_doc_items_usagehelp = df_mapitems.doc.salt_usage,
+	tiles = {"dfcaverns_salt_crystal.png"},
+	groups = {cracky = 2},
+	paramtype = "light",
+	paramtype2 = "facedir",
+	drawtype = "mesh",
+	mesh = "underch_crystal.obj",
+	light_source = 6,
+	sounds = default.node_sound_glass_defaults(),
+	use_texture_alpha = true,
+	sunlight_propagates = true,
+	on_place = df_mapitems.place_against_surface,
+})
+
+minetest.register_node("df_mapitems:salty_cobble", {
+	description = S("Salty Cobble"),
+	_doc_items_longdesc = df_mapitems.doc.salty_cobble_desc,
+	_doc_items_usagehelp = df_mapitems.doc.salty_cobble_desc,
+	tiles = {"default_cobble.png^dfcaverns_salty.png"},
+	groups = {cracky = 3, stone = 1, lava_heatable = 1},
+	_magma_conduits_heats_to = "default:cobble",
+	is_ground_content = true,
+	light_source = 2,
+	drop = 'default:cobble',
+	sounds = default.node_sound_stone_defaults(),
+})