compositor: Reparent the backend window unmanaging the display

Under X11 hiding the backend implies also unmapping the stage window, if
we do that after that we've closed the display we may end up in a
BadWindow error because such window seems to be destroyed together with
the compositor output parent (even though we are not notified about), so
to prevent this, reparent the backend window during compositor unmanage,
setting it back as a root window child.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2835
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3020>

src/tests/meson.build

@@ -684,7 +684,6 @@ if have_x11
       ],
       env: x11_env,
       suite: ['core', 'mutter/x11'],
-      should_fail: mode == 'sync',
       is_parallel: false,
       timeout: 60,
     )