Move trees into their own mod.

Tell all versions of the mapgen never to place them, and only
ever use decoration placement system.

mods/nc_terrain/decor.lua

@@ -1,22 +0,0 @@
-local minetest = minetest
-local modname = minetest.get_current_modname()
-
-minetest.register_decoration({
-		deco_type = "schematic",
-		place_on = {modname .. ":dirt_with_grass"},
-		sidelen = 16,
-		noise_params = {
-			offset = -0.008,
-			scale = 0.012,
-			spread = {x = 250, y = 250, z = 250},
-			seed = 2,
-			octaves = 3,
-			persist = 0.66
-		},
-		biomes = {"unknown"},
-		y_min = 1,
-		y_max = 31000,
-		schematic = minetest.get_modpath(modname) .. "/schematics/tree.mts",
-		flags = "place_center_x, place_center_z",
-		rotation = "random",
-	})

mods/nc_terrain/init.lua

@@ -3,10 +3,4 @@ local modname = minetest.get_current_modname()
 local path = minetest.get_modpath(modname)
 
 dofile(path .. "/node.lua")
-
-minetest.clear_registered_biomes()
-minetest.clear_registered_ores()
-minetest.clear_registered_decorations()
-
 dofile(path .. "/biome.lua")
-dofile(path .. "/decor.lua")

mods/nc_terrain/node.lua

@@ -60,9 +60,20 @@ regterrain({
 		},
 	})
 
-minetest.register_alias("mapgen_snow", "air")
-minetest.register_alias("mapgen_snowblock", "air")
-minetest.register_alias("mapgen_junglegrass", "air")
+for _, v in ipairs({
+		"snow",
+		"snowblock",
+		"junglegrass",
+		"tree",
+		"jungletree",
+		"pine_tree",
+		"leaves",
+		"apple",
+		"jungleleaves",
+		"pine_needles"
+		}) do
+	minetest.register_alias("mapgen_" .. v, "air")
+end
 
 regterrain({
 		description = "Dirt",
@@ -109,30 +120,6 @@ regterrain({
 			"desert_sand" 
 		},
 	})
-regterrain({
-		description = "Tree",
-		tiles = {
-			modname .. "_tree_top.png",
-			modname .. "_tree_top.png",
-			modname .. "_tree_side.png"
-		},
-		mapgen = {
-			"tree",
-			"jungletree",
-			"pine_tree"
-		}
-	})
-regterrain({
-		description = "Leaves",
-		drawtype = "allfaces_optional",
-		paramtype = "light",
-		mapgen = {
-			"leaves",
-			"apple",
-			"jungleleaves",
-			"pine_needles"
-		}
-	})
 
 regliquid({
 		description = "Water",

mods/nc_tree/init.lua

@@ -0,0 +1,42 @@
+local minetest = minetest
+local modname = minetest.get_current_modname()
+
+minetest.register_node(modname .. ":tree", {
+		description = "Tree",
+		tiles = {
+			modname .. "_tree_top.png",
+			modname .. "_tree_top.png",
+			modname .. "_tree_side.png"
+		}
+	})
+
+minetest.register_node(modname .. ":leaves", {
+		description = "Leaves",
+		drawtype = "allfaces_optional",
+		paramtype = "light",
+		tiles = { modname .. "_leaves.png" }
+	})
+
+minetest.register_decoration({
+		deco_type = "schematic",
+		place_on = {"nc_terrain:dirt_with_grass"},
+		sidelen = 16,
+		noise_params = {
+			offset = -0.008,
+			scale = 0.012,
+			spread = {x = 250, y = 250, z = 250},
+			seed = 2,
+			octaves = 3,
+			persist = 0.66
+		},
+		biomes = {"unknown"},
+		y_min = 1,
+		y_max = 31000,
+		schematic = minetest.get_modpath(modname) .. "/schematics/tree.mts",
+		flags = "place_center_x, place_center_z",
+		rotation = "random",
+		replacements = {
+			["nc_terrain:tree"] = modname .. ":tree",
+			["nc_terrain:leaves"] = modname .. ":leaves",
+		}
+	})