As per commit 7718e67f, destroying the compositor causes destroying window
actors and this leads to stack changes, but at this point the stack was already
disposed and cleared.
So, clear the stack when any component that could use it (compositor, and X11)
has already been destroyed.
As consequence, also the stamps should be destroyed at later point.
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/623https://gitlab.gnome.org/GNOME/mutter/merge_requests/605