Ore distribution: Deeper iron, diamond and mese block, tune gold (#2107)

Iron at y = 0 was far too easy to find.
Adjust gold lower region to be twice the depth of highest level, like
all other ores.
Diamond at y = -500 was too easy to progress to.
Make diamond and mese block deeper to create a depth progression from
mese crystal to diamond, to reflect tool progression.
This all creates a satisfying 2^n depth progression, with lower regions
being at twice the depth of highest levels.

mods/default/mapgen.lua

@@ -576,7 +576,7 @@ function default.register_ores()
 		clust_scarcity = 7 * 7 * 7,
 		clust_num_ores = 5,
 		clust_size     = 3,
-		y_max          = 0,
+		y_max          = -64,
 		y_min          = -127,
 	})
 
@@ -682,7 +682,7 @@ function default.register_ores()
 		clust_num_ores = 3,
 		clust_size     = 2,
 		y_max          = -256,
-		y_min          = -1023,
+		y_min          = -511,
 	})
 
 	minetest.register_ore({
@@ -692,77 +692,77 @@ function default.register_ores()
 		clust_scarcity = 13 * 13 * 13,
 		clust_num_ores = 5,
 		clust_size     = 3,
-		y_max          = -1024,
+		y_max          = -512,
 		y_min          = -31000,
 	})
 
-	-- Diamond
+	-- Mese crystal
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_diamond",
+		ore            = "default:stone_with_mese",
 		wherein        = "default:stone",
-		clust_scarcity = 15 * 15 * 15,
-		clust_num_ores = 4,
+		clust_scarcity = 14 * 14 * 14,
+		clust_num_ores = 5,
 		clust_size     = 3,
 		y_max          = 31000,
 		y_min          = 1025,
 	})
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_diamond",
+		ore            = "default:stone_with_mese",
 		wherein        = "default:stone",
-		clust_scarcity = 17 * 17 * 17,
-		clust_num_ores = 4,
-		clust_size     = 3,
+		clust_scarcity = 18 * 18 * 18,
+		clust_num_ores = 3,
+		clust_size     = 2,
 		y_max          = -512,
 		y_min          = -1023,
 	})
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_diamond",
+		ore            = "default:stone_with_mese",
 		wherein        = "default:stone",
-		clust_scarcity = 15 * 15 * 15,
-		clust_num_ores = 4,
+		clust_scarcity = 14 * 14 * 14,
+		clust_num_ores = 5,
 		clust_size     = 3,
 		y_max          = -1024,
 		y_min          = -31000,
 	})
 
-	-- Mese crystal
+	-- Diamond
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_mese",
+		ore            = "default:stone_with_diamond",
 		wherein        = "default:stone",
-		clust_scarcity = 14 * 14 * 14,
-		clust_num_ores = 5,
+		clust_scarcity = 15 * 15 * 15,
+		clust_num_ores = 4,
 		clust_size     = 3,
 		y_max          = 31000,
 		y_min          = 1025,
 	})
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_mese",
+		ore            = "default:stone_with_diamond",
 		wherein        = "default:stone",
-		clust_scarcity = 18 * 18 * 18,
-		clust_num_ores = 3,
-		clust_size     = 2,
-		y_max          = -512,
-		y_min          = -1023,
+		clust_scarcity = 17 * 17 * 17,
+		clust_num_ores = 4,
+		clust_size     = 3,
+		y_max          = -1024,
+		y_min          = -2047,
 	})
 
 	minetest.register_ore({
 		ore_type       = "scatter",
-		ore            = "default:stone_with_mese",
+		ore            = "default:stone_with_diamond",
 		wherein        = "default:stone",
-		clust_scarcity = 14 * 14 * 14,
-		clust_num_ores = 5,
+		clust_scarcity = 15 * 15 * 15,
+		clust_num_ores = 4,
 		clust_size     = 3,
-		y_max          = -1024,
+		y_max          = -2048,
 		y_min          = -31000,
 	})
 
@@ -786,8 +786,8 @@ function default.register_ores()
 		clust_scarcity = 36 * 36 * 36,
 		clust_num_ores = 3,
 		clust_size     = 2,
-		y_max          = -1024,
-		y_min          = -2047,
+		y_max          = -2048,
+		y_min          = -4095,
 	})
 
 	minetest.register_ore({
@@ -797,7 +797,7 @@ function default.register_ores()
 		clust_scarcity = 28 * 28 * 28,
 		clust_num_ores = 5,
 		clust_size     = 3,
-		y_max          = -2048,
+		y_max          = -4096,
 		y_min          = -31000,
 	})
 end