llvmpipe/lavapipe: anisotropic texture filtering
In order to expose OpenGL 4.6 the last missing feature in llvmpipe is anisotropic texture filtering. Adding support for this also allows lavapipe expose the Vulkan samplerAnisotropy feature. I started writing anisotropic support > 6 months ago. At the time we were trying to deprecate the classic swrast driver, and someone pointed out it had support for anisotropic filtering. This support had also been ported to the softpipe driver, but never to llvmpipe. I had also considered porting swiftshaders anisotropic support, but since I was told the softpipe code was functional and had users I based my llvmpipe port on that. Porting the code to llvmpipe means rewriting it to generate LLVM IR using the llvmpipe vector processing code. This is a lot messier than just writing linear processing code, and when I thought I had it working it passes GL CTS, but failed the VK CTS. The results also to my eye looked worse than I'd have thought was acceptable, and softpipe seemed to be as bad. Once...