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) e6a13e5d57
("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

@@ -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,