include top textures for MeseCraft and Minetest Game

so it doesn't look too horrible when joining a server with its own texture pack.
See issue #1

default_snow.png is renamed to mtgdefault_snow.png and applied via override.txt to avoid potential complications/conflicts with the MineClone games, which still use a "default_snow.png" file containing a different texture.

override.txt

@@ -1,3 +1,4 @@
 mcl_core:dirt_with_grass_snow top mcldefault_snow.png
 mcl_core:mycelium_snow top mcldefault_snow.png
 mcl_core:podzol_snow top mcldefault_snow.png
+default:dirt_with_snow top mtgdefault_snow.png