Avoids using a static and make sure things are properly freed by
using g_autoptr. Also take the opportunity to add some missing NULL
initialization for auto-pointers variables
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3687>
Fullscreen Wayland toplevel surfaces don't need to respect the
configured size in which case the window content get centered on a black
background which covers the whole monitor.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2338>