Primordial cavern layer (#12)

* bring in the art assets from ClockGen's "better_caves_modpack" under CC BY 4.0,, code written from scratch.

* update mapgen_helper

* import ClockGen's giant mushroom schematics, make them and giant ferns growable

* add giant jungle mushroom, rough out actual cavern layer code framework

* fungal ceiling decorations

* decorate fungal floor a bit

* update mapgen_helper

* update primordial mushroom schematic placement to ensure it fits

* add giant mycelium fungoidal structure

* add giant mycelium to mapgen

* fix settings for giant mycelium

* make mycelium grow when players aren't present

* allow mycelium growth to pause when it hits unloaded areas

* add a use for giant mycelium

* make giant mushrooms edible, make jungle trees growable

* rough out the jungle biome

* Make a spectrum of jungle growth

* optimize pngs, add is_ground_content to everything

* use custom is_ground_content method

* fix a crash with veinstone, and simplify nvals_cave lookup now that overgen covers the same area

* more fixes for overgen support

* remove unintentional airspace from underside of slade

* fix for overgen crash in level 2

* primordial column material, add sealed shafts to underworld

* add seal block

* Set up puzzle seals to be able to dig a staircase shaft through the slade layer. TODO: the puzzle to guard the trigger.

* puzzle seals now fully functional. Need to add clues for decoding the keys next.

* add a small bit of anti-griefing - the seal breach bell only tolls globally 13 times per activation

* add flowers to the underworld warrior bones

* switch to a different key bell

* fancy up the puzzle seal temples with some custom blocks, add sub-slade grid passages

* add a clue to the seal formspec

* tweak background of inscription 2 so it's less obviously a copy of the background for inscription 1

* switch to compositing to save a few bytes

* fancy up the seal's upper surface with inscriptions to make the formspec feel consistent

* puzzle particle, bones were only spawning on top of structures

* fix ice/oil on level 3, tweak some loot probabilities

* add trail mod support

* remove deprecated files

* boost default plant growth delay, add growing selection boxes

* update map colours

* add named waypoints to the underworld

* try a more efficient way of changing the interiors of columns

* polishing up the Primordial layer

* update guide with some Primordial teasers

* updated magma sea screenshot

* update mapgen_helper and subterrane

* reduce density of megaflora a bit - was too hard to walk through

* spreading_dirt_type depends on light, create my own ABM instead

* add names to the glowing pits and some of the ruins

* separate setting for ruin markers

* record identity of slade-breachers

* make mycelia climbable

* update subterrane

* change surface tunnel detection to allow above-ground stalactites and stalagmites

* add rare thicker Goblin Caps, suitable for use as huts.

* better goblin cap schematics

* update colours

* make it slightly harder to dig down through amethyst sheathing of pits

* fixing up fungus light sensitivity, tree growth code

* fix a few minor bugs

* update deprecated functions

* add various eating sounds

* make mapping kit requirement more flexible

* update spindlestem growth code, remove deprecated functions

* fix leftover undefined variable

* add fireflies to primordial, spread out the post-mapgen node timer for plant matter a bit more.

* fix bones formspec

* add lbm to upgrade old bones

* fix slade undiggability

* make torchspines smokey and manually lightable

* fix drop definitions

* generate dry stalactites in near-surface caverns.

* caverns become far too smokey, alas

* add pitter patter of spore tree spores, alternate paper recipe

* new mapgen_helper metrics

* add smokey back to torchspine now that it can be dialed down a bit

* replace glowstone texture with a new animated one

* switch from ABM to node timer for mapgen mycelium growth

* make mapgen mycelium timer delay configurable

* improve the efficiency of giant mycelium growth using flat node array, fewer dereferences

* remove the smoke from torchspines again - it doesn't dissipate that deep underground

* give slade a more muted, gloomy hue to differentiate it from nether stone

* update screenshots with new slade colors

* update mapgen_helper

df_primordial_items/textures/license.txt

@@ -0,0 +1,10 @@
+All primordial item textures not explicitly listed here were created by ClockGen and licensed under the CC BY 4.0
+
+Created by FaceDeer and also licened under CC BY 4.0 (as well as CC0 public domain):
+dfcaverns_mush_diced_giant_mushroom.png
+dfcaverns_mush_giant_hypha.png
+dfcaverns_mush_sapling.png
+dfcaverns_mush_mycelial_fibers.png
+
+From Minetest default mod:
+dfcaverns_jungle_sapling.png