Applied patch from bug #871

	Applied patch from bug #871

	* clutter/x11/clutter-backend-x11.c (clutter_backend_x11_dispose):
	Call g_slist_foreach instead of iterating over the stage_manager
	list manually when deleting stages. Otherwise the 'next' pointer
	of the list node can get corrupted when the actor removes itself
	from the list.

	* clutter/clutter-stage.c (clutter_stage_dispose): Call
	clutter_actor_unrealize in the dispose handler. This fixes
	problems where the dispose handler for the ClutterStageWrapper
	can't deselect the GL context until the stage is unrealized.