If the parent of a transient is not a native Wayland window (e.g.
offscreen window), the transient loop check will crash.
Check for the actual type in the transient loop check and do not assume
the parent is necessarily Wayland native.
bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=761156
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>