llvmpipe: Code generate the texture sampling inside the shader.

llvmpipe: Code generate the texture sampling inside the shader.

Finally a substantial performance improvement: framerates of apps using
texturing tripled, and furthermore, enabling/disabling texturing only
affects around 15% of the framerate, which means the bottleneck is now
somewhere else.

Generated texture sampling code is not complete though -- we always
sample from the base level -- so final figures will be different.