Mutter needs to fetch the X11 Window ID from the onscreen and did that
by using an X11 specific API on the CoglOnscreen, where the X11 type was
"expanded" (Window -> uint32_t). Change this by introducing an interface
called CoglX11Onscreen, implemented by both the Xlib and GLX onscreen
implementations, that keeps the right type (Window), while avoiding X11
specific API for CoglOnscreen.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1514>
