Add biomes-based implementation of the mapgen, and decorations

Prevents the c++/native mapgen from placing ores or decorations (when useBiomes is true).
The biomes-based implementation is faster, more deterministic, and keeps its dungeons, but requires MT 5.1 features to work.

Also
 * Glowstone & Netherrack stalactites
 * Include the new decorations in mapgen_nobiomes
 * Decorate dungeons - Add windows and glowstone "chandeliers" to some rooms
 * Configurable Nether floor and ceiling depths

mapgen_decorations.lua

@@ -0,0 +1,138 @@
+--[[
+
+  Register decorations for Nether mapgen
+
+  Copyright (C) 2020 Treer
+
+  Permission to use, copy, modify, and/or distribute this software for
+  any purpose with or without fee is hereby granted, provided that the
+  above copyright notice and this permission notice appear in all copies.
+
+  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+  WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+  WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR
+  BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
+  OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+  WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+  ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
+  SOFTWARE.
+
+]]--
+
+-- Lava is unreliable in the old Nether mapgen because it removes lava
+-- from the overdraw areas, so any decorations involving lava will often
+-- have the lava missing depending on whether nearby chunks were already
+-- emerged or not before the decoration was placed.
+local allow_lava_decorations = nether.useBiomes
+
+local _ = {name = "air",                     prob = 0}
+local A = {name = "air",                     prob = 255, force_place = true}
+local G = {name = "nether:glowstone",        prob = 255, force_place = true}
+local N = {name = "nether:rack",             prob = 255}
+local S = {name = "nether:sand",             prob = 255, force_place = true}
+local L = {name = "default:lava_source",     prob = 255, force_place = true}
+
+
+-- =================
+--    Stalactites
+-- =================
+
+local schematic_GlowstoneStalactite = {
+    size = {x = 5, y = 10, z = 5},
+    data = {
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, N, G, N, _,
+        _, N, N, N, _,
+
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, G, _, _,
+        _, _, G, _, _,
+        _, G, G, G, _,
+        N, G, G, G, N,
+        N, N, G, N, N,
+
+        _, _, N, _, _,   -- ypos 0, prob 25% (64/256)
+        _, _, G, _, _,   -- ypos 1, prob 37% (96/256)
+        _, _, G, _, _,   -- ypos 2, prob 100%
+        _, _, G, _, _,   -- ypos 3, prob 100%
+        _, _, G, G, _,   -- ypos 4, prob 50% (128/256) to make half of stalactites asymmetric
+        _, G, G, G, _,   -- ypos 5, prob 75% (192/256)
+        _, G, G, G, _,   -- ypos 6, prob 75% (192/256)
+        _, G, G, G, _,   -- ypos 7, prob 100%
+        G, G, G, G, G,   -- ypos 8, prob 100%
+        N, G, G, G, N,   -- ypos 9, prob 75% (192/256)
+
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, G, _, _,
+        _, _, G, _, _,
+        _, _, G, _, _,
+        _, G, G, G, _,
+        N, G, G, G, N,
+        N, N, G, N, N,
+
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, _, _, _, _,
+        _, N, G, N, _,
+        _, N, N, N, _
+    },
+    -- Y-slice probabilities do not function correctly for ceiling schematic
+    -- decorations because they are inverted, so ypos numbers have been inverted
+    -- to match, and a larger offset in place_offset_y should be used (e.g. -3).
+    yslice_prob = {
+        {ypos = 9, prob = 192},
+        {ypos = 6, prob = 192},
+        {ypos = 5, prob = 192},
+        {ypos = 4, prob = 128},
+        {ypos = 1, prob = 96},
+        {ypos = 0, prob = 64}
+    }
+}
+
+minetest.register_decoration({
+    name = "Glowstone stalactite",
+    deco_type = "schematic",
+    place_on = "nether:rack",
+    sidelen = 80,
+    fill_ratio = 0.0004,
+    biomes = {"nether_caverns"},
+    y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
+    y_min = nether.DEPTH_FLOOR,
+    schematic = schematic_GlowstoneStalactite,
+    flags = "place_center_x,place_center_z,force_placement,all_ceilings",
+    place_offset_y=-3
+})
+
+minetest.register_decoration({
+    name = "Netherrack stalactite",
+    deco_type = "schematic",
+    place_on = "nether:rack",
+    sidelen = 80,
+    fill_ratio = 0.0007,
+    biomes = {"nether_caverns"},
+    y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
+    y_min = nether.DEPTH_FLOOR,
+    schematic = schematic_GlowstoneStalactite,
+    replacements = {["nether:glowstone"] = "nether:rack"},
+    flags = "place_center_x,place_center_z,all_ceilings",
+    place_offset_y=-3
+})