Allow players to create custom texture packs using owned packs

So basically, a lot of the newer looks given to things (Mossy Cobble, Zombie Pigmen, Ores) are blatant downgrades from the classic  textures, while others (Lava, Wheat, Paths) are way better than they used to be, so why not let us mix and match textures from our owned packs, including ones we got from the store, there's literally no downside, just make it so that you choose a base texture pack and edit specific blacks or mobs like you would with a skin.