Tin: Tune mapgen, bugfix, fix texture credits

Previously i had matched ore density to moreores, but this density was
too high and out of balance with MTGame, reduce density to be similar
to copper ore.
The 2 levels of underground ore were overlapping, fix.
Textures are actually by kilbith, update credits.

Also credit kilbith for bronze, copper, gold and steel blocks, credits
had not been updated at the time.

mods/default/mapgen.lua

@@ -364,7 +364,7 @@ function default.register_ores()
 		ore            = "default:stone_with_tin",
 		wherein        = "default:stone",
 		clust_scarcity = 10 * 10 * 10,
-		clust_num_ores = 9,
+		clust_num_ores = 5,
 		clust_size     = 3,
 		y_min          = 1025,
 		y_max          = 31000,
@@ -375,9 +375,9 @@ function default.register_ores()
 		ore            = "default:stone_with_tin",
 		wherein        = "default:stone",
 		clust_scarcity = 13 * 13 * 13,
-		clust_num_ores = 7,
+		clust_num_ores = 4,
 		clust_size     = 3,
-		y_min          = -31000,
+		y_min          = -127,
 		y_max          = -32,
 	})
 
@@ -386,7 +386,7 @@ function default.register_ores()
 		ore            = "default:stone_with_tin",
 		wherein        = "default:stone",
 		clust_scarcity = 10 * 10 * 10,
-		clust_num_ores = 9,
+		clust_num_ores = 5,
 		clust_size     = 3,
 		y_min          = -31000,
 		y_max          = -128,