RFE: Enhance drisw to work on displays without a GLX extension
(Prompted by #3808 (closed))
One of the few advantages of the xlib targets is they work against servers that don't have a GLX extension. There's no real reason we couldn't do the same thing in the real libGL as well. You'd only have one GLX visual, so it would need to have pretty much every feature enabled, but other than that things should work just as well as they do with GLX+llvmpipe or with the xlib target.