Honor GLX_SAMPLE_BUFFERS and GLX_SAMPLES.
Pbuffers on DSPL side are never used so relax their format there.

Also ignore GLX_X_RENDERABLE as we have explicit GLX_DRAWABLE_TYPE.

This makes pass visualinfo utility from GLEW library.
Pbuffers on DSPL side are never used so allocate them as 1x1 and never swap.