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 I swung back around to this I decided to port the VK CTS test to GL and run it on softpipe and llvmpipe code. Initially llvmpipe had some more bugs to solve esp where the mipmap levels were being chosen, but once I'd finished aligning softpipe and llvmpipe I started digging into why the softpipe code wasn't as nice as I expected.

The softpipe code was based on an implementation of an Elliptical Weighted Average Filter (EWA). The paper "Creating Raster Omnimax Images from Multiple Perspective Views Using the Elliptical Weighted Average Filter" described this. I sat down with the paper and softpipe code and eventually found the one line where they diverged.[1] This turned out to be a bug introduced in a refactoring 5 years ago, and nobody had noticed or tracked it down.

I then ported the same fix to my llvmpipe code, and VK CTS passes. I also optimized the llvmpipe code a bit to avoid doing pointless sampling and cleaned things up. This code landed in [2] today.

For GL4.6 there are still some fixes in other areas.

[1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11917

[2] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8804

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