OSX: add multistage support

OSX: add multistage support

	Bug #911 - OSX: add multistage support

	* clutter/osx/clutter-backend-osx.{c,h}
	(clutter_backend_osx_init_stage, clutter_backend_osx_get_stage,
	clutter_backend_osx_redraw, clutter_backend_osx_create_stage,
	clutter_backend_osx_ensure_context, clutter_backend_osx_class_init,
	clutter_backend_osx_dispose, ClutterGLView:drawRect:):
	* clutter/osx/clutter-stage-osx.{c,h} (clutter_stage_osx_realize,
	ClutterGLWindow:setFrameSize:):
	Adapt to new multistage backend API. Don't keep a pointer to
	default stage. Derive from ClutterActor instead of ClutterStage.
	Implement ClutterStageWindow interface. Paint, resize and
	otherwise manipulate the wrapper rather than self when necessary.

	(clutter_backend_post_parse): Create our singleton GL context
	here. We could probably create the context when the default
	stage is created, but I think this is more clean.

	* clutter/osx/clutter-event-osx.c (clutter_event_osx_translate)
	* clutter/osx/clutter-stage-osx.c (clutter_stage_osx_state_update,
	ClutterGLWindow:windowShouldClose:):
	* clutter/osx/clutter-stage-osx.h: Export ClutterGLWindow interface
	for clutter-event-osx.c to easily get the stage for NSWindow.
	Fill in ClutterEventAny::stage on our events.

	Consistently use 'stage_osx' and 'wrapper' as variable names
	when referring to ClutterStageOSX and ClutterStage objects
	respectively.