Reduce lava in biomes-based mapgen (#25)

The biomes-based mapgen was creating the same amount of lava as the original nether mapgen, but it doesn't have the same issue of chunk emerge order sometimes causing lava in the overdraw regions to get removed. This adjustment will hopefully balance that a little.

Also makes glowstone stalactite a bit rarer.

Adjusting lava ore scarcity in the biomes-mapgen doesn't cause forwards or backwards compatibilty issues with existing maps, likewise with schematic rarety like glowstone stalactites, so can afford to fiddle and tune.

mapgen.lua

@@ -113,7 +113,7 @@ override_underground_biomes()
 
 -- nether:native_mapgen is used to prevent ores and decorations being generated according
 -- to landforms created by the native mapgen.
--- Ores and decorations are registered against "nether:rack" instead, and the lua
+-- Ores and decorations can be registered against "nether:rack" instead, and the lua
 -- on_generate() callback will carve the Nether with nether:rack before invoking
 -- generate_decorations and generate_ores.
 minetest.register_node("nether:native_mapgen", {})
@@ -160,7 +160,7 @@ minetest.register_ore({
 	ore_type       = "scatter",
 	ore            = "default:lava_source",
 	wherein        = "nether:rack",
-	clust_scarcity = 32 * 32 * 32,
+	clust_scarcity = 36 * 36 * 36,
 	clust_num_ores = 4,
 	clust_size     = 2,
 	y_max = NETHER_CEILING,

mapgen_decorations.lua

@@ -113,7 +113,7 @@ minetest.register_decoration({
     deco_type = "schematic",
     place_on = "nether:rack",
     sidelen = 80,
-    fill_ratio = 0.0004,
+    fill_ratio = 0.0003,
     biomes = {"nether_caverns"},
     y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
     y_min = nether.DEPTH_FLOOR,
@@ -127,7 +127,7 @@ minetest.register_decoration({
     deco_type = "schematic",
     place_on = "nether:rack",
     sidelen = 80,
-    fill_ratio = 0.0007,
+    fill_ratio = 0.0008,
     biomes = {"nether_caverns"},
     y_max = nether.DEPTH_CEILING, -- keep compatibility with mapgen_nobiomes.lua
     y_min = nether.DEPTH_FLOOR,