GPipe: Binding textures to geometry

I'm by no means an GL expert, but I think it should be possible to
attach a texture index to vertices, so that only one fragment shader is
needed instead of one for every texture. Currently, I'm duplicating the
whole graphics pipeline for virtually every triangle, and it seems that
is what's killing performance.


Random minor nitpick: The matrix multiplications in the example are done
on the CPU, not GPU.

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