Option of combining Texture Packs

Basically, if there is a texture pack that for example, only changes mobs and another that only changes items etc. we should have an ability to combine them.

As long as the two texture packs don't have the same textures changed, they should be compatible to use at the same time.

We can just select them both (even currently, we can have many texture packs ready, but only one in use) and it would pull textures from both of them.