wip: softpipe: Implement GL_ARB_sample_shading
We'll hit GL 4.0 yet.
This should refer back to the corresponding llvmpipe changes you mimicked. And it doesn't really work yet because softpipe does not in fact implement multisampling, unlike llvmpipe, so the tests get confused. Some of that I think means the Xvfb in the softpipe tests needs to also force softpipe as the driver so you don't get ms visuals? Or we should just go ahead and implement ms in softpipe?