gallium: Permit surface_copy and surface_fill to be NULL.

gallium: Permit surface_copy and surface_fill to be NULL.

Uf. Lots of files touched. Would people with working vega, xorg, dri1, etc.
please make sure you are not broken, and fix yourself up if you are.

There were only two or three places where the code did not have painful
fallbacks, so I would advise st maintainers to find less painful workarounds,
or consider overhauling util_surface_copy and util_surface_fill.

Per ymanton, darktama, and Dr_Jakob's suggestions, clear has been left as-is.

I will not add PIPE_CAP_BLITTER unless it is deemed necessary.