asahi: Core Gallium driver support for images
This adds the core Gallium driver support for packing Pixel BackEnd (PBE) descriptors for pipe_image_view
s, setting up the bare minimum data structures needed to bind images for shader writes. This does not include the corresponding compiler support, which will come next. I wanted to break off a manageable chunk for review and merging this as-is won't hurt anything.
Cc @eric