x11: Store the EGL config in the display

x11: Store the EGL config in the display

We only have one config, because we use the same Visual everywhere.
Store this config in the GdkDisplayX11 struct for easy access.

Also do this on initialize, because if creating the config fails, we
want to switch to GLX instead of failing to do GL at all.

This also simplifies a lot of code as we can share Visual, Colormap, etc
across surfaces.