stage: Pass paint context in phase callbacks

If there is a paint context available (i.e. for the phases that are
during the actual stage paint), pass it along the callbacks, so that
the callback implementations can change their operation depending on the
paint context state.

This also means we can get the current view from the paint context,
instead of the temporarily used field in the instance struct.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1207

src/backends/meta-screen-cast-monitor-stream-src.c

@@ -115,9 +115,10 @@ meta_screen_cast_monitor_stream_src_get_specs (MetaScreenCastStreamSrc *src,
 }
 
 static void
-stage_painted (MetaStage        *stage,
-               ClutterStageView *view,
-               gpointer          user_data)
+stage_painted (MetaStage           *stage,
+               ClutterStageView    *view,
+               ClutterPaintContext *paint_context,
+               gpointer             user_data)
 {
   MetaScreenCastStreamSrc *src = META_SCREEN_CAST_STREAM_SRC (user_data);