zink: support fbfetch
I thought I made a ticket for this, but I guess not? Anyway, we should probably do this at some point. I started looking at it some time ago for fun, considering variations on attaching an image (both directly attaching the framebuffer and blitting to a new one) for an injected storage image descriptor (with accompanying nir pass to load the pixel data for the fragment at gl_FragCoordMESA
).
I'm not sure if this is how we want to end up doing it, but it's a useful feature, so we should at some point work on it.