[mutter] stage: Only invoke paint phase callbacks when painting views

[Jonas Ådahl <jadahl src gnome org>]

commit 90c4b6492fc227d59a53d6fa0b7c1cb602c52ca8
Author: Jonas Ådahl <jadahl gmail com>
Date:   Mon Apr 20 22:39:08 2020 +0200

    stage: Only invoke paint phase callbacks when painting views
    
    These phase callbacks are not intended to be inovked when something
    secondary is painting the stage, such as a screen cast stream, or
    similar. Thus, only invoke the callbacks when there is a view associated
    with the paint context, which will not be the case for offscreen
    painting.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1207

 src/backends/meta-stage.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)
---
diff --git a/src/backends/meta-stage.c b/src/backends/meta-stage.c
index d5a5d8720..e5f156880 100644
--- a/src/backends/meta-stage.c
+++ b/src/backends/meta-stage.c
@@ -198,17 +198,22 @@ meta_stage_paint (ClutterActor        *actor,
   CLUTTER_ACTOR_CLASS (meta_stage_parent_class)->paint (actor, paint_context);
 
   view = clutter_paint_context_get_stage_view (paint_context);
-
-  notify_watchers_for_mode (stage, view, paint_context,
-                            META_STAGE_WATCH_AFTER_ACTOR_PAINT);
+  if (view)
+    {
+      notify_watchers_for_mode (stage, view, paint_context,
+                                META_STAGE_WATCH_AFTER_ACTOR_PAINT);
+    }
 
   g_signal_emit (stage, signals[ACTORS_PAINTED], 0);
 
   for (l = stage->overlays; l; l = l->next)
     meta_overlay_paint (l->data, paint_context);
 
-  notify_watchers_for_mode (stage, view, paint_context,
-                            META_STAGE_WATCH_AFTER_OVERLAY_PAINT);
+  if (view)
+    {
+      notify_watchers_for_mode (stage, view, paint_context,
+                                META_STAGE_WATCH_AFTER_OVERLAY_PAINT);
+    }
 }
 
 static void