Draft: gpu: add support for pl_tex_params.mipmaps

I'm not very happy with this approach for three reasons:

1. Our mipmap generation code is terrible (needs gamma correction, needs better handling of npot2 textures)
2. Currently no real way to actually use mipmaps
3. Actually caching mipmaps between frames (as would be the goal for perf) is too costly in terms of API, and is basically impossible to do zerocopy unless we also patch all of ffmpeg to generate mipmaps
4. It might make more sense to just generate low-res intermediate textures on demand, if needed to anti-alias to extreme ratios (basically turning normal downscalers into cascading downscalers)

In conclusion, I think this feature is probably dead on arrival. Still, it was interesting while it lasted.