GL: Set up the textures used for each draw call.

Each Maxwell shader stage can have an arbitrary number of textures, but we're limited to a certain number in OpenGL. We try to only use the minimum amount of host textures by not keeping a 1:1 relation between guest texture ids and host texture ids, ie, guest texture id 8 can be host texture id 0 if it's the only texture used in the guest shader program.
This mapping will have to be passed to the shader decompiler so it can rewrite the texture accesses.