* clutter/sdl/clutter-stage-sdl.[ch]: Port the SDL stage to
the new stage implementation API.
* clutter/sdl/clutter-backend-sdl.[ch]: Port the SDL backend
to the new backend API.
* clutter/sdl/clutter-event-sdl.c:
(clutter_event_dispatch): Assign the default stage as the
origin of the event.
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