When running xfwm4 from a nested X11 server (Xnest, Xephyr) from within
a Wayland session, GDK will selects in priority the Wayland backend,
which will lead to a crash because xfwm4 is an X11 window manager and
not a Wayland client.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>