WIP: glx: Implement GLX_EXT_context_priority
Just the GLX version of EGL_IMG_context_priority.
Not quite complete. The spec says there's a query for the actual context priority you got, but we don't implement that yet. Probably that's worth preserving for symmetry with EGL, so either the spec needs to be updated or (preferably) we need a way to get the real priority out of the created context.