asahi: gl_FragDepth, stencil export, seamful cubemaps, anisotropy, many assorted bug fixes

The latest batch of Asahi changes, mix of Lina and me here. Most of this is pulled from asahi/mesa so well tested at this point, a few commits are pulled from agx/next so might be spicier. Cc @asahilina

There's a new NIR intrinsic added for depth/stencil export hence the tag.