[Mesa-dev] ARB_framebuffer_no_attachment for radeonsi,r600g,nvc0

[Edward O'Callaghan]

Due to popular demand this _should_ implement this GL extension
correctly now and include the various community feedback. Sorry
for this taking so very long because of external real-life
pressures.. In any case here it is.

This passes piglit fully on nvc0 (checked awhile ago); on
radeonsi today with ssbo/image and finally with r600g without
ssbo/image support. There are probably things that make this
miss the "pefect" mark, however this works. I also like to
thank Bas Nieuwenhuizen for helping me think though some issues
I had and the collaboration there!