gallium/hud: fixes for lima

I'm submitting these now also with feedback in mind as I'm not sure if either of those fixes are the correct ones.

gallium hud has always been very broken in lima and now I finally took some time to debug why that is the case.

There may also be some followup fixes in lima. We could probably declare support for PIPE_FORMAT_B8G8R8X8_SRGB but then I still need to figure out if we need something else to actually support that. We may have to further ensure that the varying layout matches when assembling the fragment shader commands but so far it has never been an issue with any application before. I suppose because when shaders go all the way from glsl, some linker optimization ensures that the varying layout matches.

With the two patches gallium hud works perfectly with lima. There is some more information in the commit logs.

Any thoughts?