[mutter] screen-cast/area-src: Add before-paint watch to catch scanouts

[Marge Bot <marge-bot src gnome org>]

commit 8de0771aad02cce7f69e27d37a4922583010c07d
Author: Michel Dänzer <mdaenzer redhat com>
Date:   Fri Jun 25 12:54:46 2021 +0200

    screen-cast/area-src: Add before-paint watch to catch scanouts
    
    Scanouts are taken away after painting. However, when we're
    streaming, what we actually want is to capture whatever is
    going to end up on screen - and that includes the scanout
    if there's any.
    
    Add a before-paint watch that only records new frames if a
    scanout is set.
    
    Inspired by (and commit log mostly copied from) e6a13e5d5774
    ("monitor-stream-src: Add before-paint watch to catch scanouts").
    
    v2:
    * Do not call stage_painted from before_stage_painted (Georges Basile
      Stavracas Neto)
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1707
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1902>

 src/backends/meta-screen-cast-area-stream-src.c | 27 +++++++++++++++++++++++++
 1 file changed, 27 insertions(+)
---
diff --git a/src/backends/meta-screen-cast-area-stream-src.c b/src/backends/meta-screen-cast-area-stream-src.c
index 96953fe7f9..aa15481010 100644
--- a/src/backends/meta-screen-cast-area-stream-src.c
+++ b/src/backends/meta-screen-cast-area-stream-src.c
@@ -231,6 +231,25 @@ maybe_record_frame_on_idle (gpointer user_data)
   return G_SOURCE_REMOVE;
 }
 
+static void
+before_stage_painted (MetaStage           *stage,
+                      ClutterStageView    *view,
+                      ClutterPaintContext *paint_context,
+                      gpointer             user_data)
+{
+  MetaScreenCastAreaStreamSrc *area_src =
+    META_SCREEN_CAST_AREA_STREAM_SRC (user_data);
+  MetaScreenCastStreamSrc *src = META_SCREEN_CAST_STREAM_SRC (area_src);
+
+  if (area_src->maybe_record_idle_id)
+    return;
+
+  if (!clutter_stage_view_peek_scanout (view))
+    return;
+
+  area_src->maybe_record_idle_id = g_idle_add (maybe_record_frame_on_idle, src);
+}
+
 static void
 stage_painted (MetaStage           *stage,
                ClutterStageView    *view,
@@ -293,6 +312,14 @@ add_view_painted_watches (MetaScreenCastAreaStreamSrc *area_src)
         {
           MetaStageWatch *watch;
 
+          watch = meta_stage_watch_view (meta_stage,
+                                         CLUTTER_STAGE_VIEW (view),
+                                         META_STAGE_WATCH_BEFORE_PAINT,
+                                         before_stage_painted,
+                                         area_src);
+
+          area_src->watches = g_list_prepend (area_src->watches, watch);
+
           watch = meta_stage_watch_view (meta_stage,
                                          CLUTTER_STAGE_VIEW (view),
                                          META_STAGE_WATCH_AFTER_ACTOR_PAINT,