xwayland: Add EGL-backed GLX provider

Without this we're using driswrast to set up GLX visuals. This is unfortunate because llvmpipe does not expose multisample configs, so various apps that expect them will fail. With this we just query the capabilities of the EGL that's backing glamor, and reflect that to the GLX clients. This also paves the way for xserver to stop being a DRI driver loader, which is nice.

Signed-off-by: Adam Jackson ajax@redhat.com