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.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3283>

src/tests/meta-ref-test.c

@@ -270,11 +270,11 @@ typedef struct
 } CaptureViewData;
 
 static void
-on_after_paint (MetaStage           *stage,
-                ClutterStageView    *view,
-                ClutterPaintContext *paint_context,
-                ClutterFrame        *frame,
-                gpointer             user_data)
+on_after_paint (MetaStage            *stage,
+                ClutterStageView     *view,
+                const cairo_region_t *redraw_clip,
+                ClutterFrame         *frame,
+                gpointer              user_data)
 {
   CaptureViewData *data = user_data;
   MtkRectangle rect;