* clutter/clutter-actor.c (clutter_actor_destroy): Bail out
if clutter_actor_destroy() was called on the stage: the stage
is not for the user to destroy.
* clutter/x11/clutter-backend-x11.c:
* clutter/eglnative/clutter-backend-egl.c:
* clutter/sdl/clutter-backend-sdl.c:
* clutter/osx/clutter-backend-osx.c: Unset the top-level private
flag on the stage when disposing it, so the backends can safely
call clutter_actor_destroy().
* clutter/clutter-private.h: Tweak the private flags accessors,
to avoid the typecheck.