Implement GLX_EXT_no_config_context

This is the GLX counterpart to EGL_KHR_no_config_context, which lets you create a context that's compatible with every fbconfig and avoid needing to pick exactly one to be compatible with. I'd tried to merge this ages ago, but reverted it because it provoked unrelated client-side bugs, which are now fixed.

The trick then is you need to be able to work backwards from the current drawable to its fbconfig, and the way we get that from the server provokes server bugs if it hasn't actually set up a context-drawable binding, which it would normally do on MakeCurrent, but uh, the DRI code kinda sorta forgot to ever send that request. So the first patch tries very carefully to send that request only for servers it thinks are new enough. It ought to fix a bunch of piglits too, but this all predates our CI so I'm sure this'll fail on the first pipeline run and will need another revision.

cc @kwg @idr who had reviewed the enablement (second) patch the first time.