screen-cast/stream: Pass redraw clip to stage watches instead of paint context

The virtual stream source with CURSOR_MODE_EMBEDDED uses META_STAGE_WATCH_AFTER_PAINT as the callback for recording its frame. In this stage of the paint though, there is no ClutterPaintContext anymore (there only is a paint context during the paint, not afterwards). The callback (actors_painted()) tries to get the redraw clip from the paint context, and we end up with a NULL pointer crash.

We actually do still have a redraw clip at this point, so because everyone uses the paint context to get the redraw clip anyway, just pass the redraw clip to the stage watches directly.