gallivm: anisotropic filtering
It seems like this could be handled by looping over the lp_bld_sample_soa.c:3249 cases 1 < ANISO_COUNT
times, applying a factor to the UV coords, and then scaling the sample results back into sampled texel? Swiftshader does this using tables of constants for the aniso factor, and it doesn't look particularly complex to mimic.