screen-cast/monitor-stream: Don't fall apart when monitor changes

If the monitor configuration changed, even though the streamed monitor
didn't change, we'd still fail to continue streaming, as we failed to
update the stage watchers, meaning we wouldn't be notified about when
the stage views were painted.

Fix this by reattaching the stage watches, i.e. update the painted
signalling listeners to listen to the right views, when monitor changes
happens.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1691>

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

@@ -323,18 +323,59 @@ add_view_watches (MetaScreenCastMonitorStreamSrc *monitor_src,
     }
 }
 
+static void
+reattach_watches (MetaScreenCastMonitorStreamSrc *monitor_src)
+{
+  MetaScreenCastStreamSrc *src = META_SCREEN_CAST_STREAM_SRC (monitor_src);
+  MetaScreenCastStream *stream;
+  ClutterStage *stage;
+  GList *l;
+
+  stream = meta_screen_cast_stream_src_get_stream (src);
+  stage = get_stage (monitor_src);
+
+  for (l = monitor_src->watches; l; l = l->next)
+    meta_stage_remove_watch (META_STAGE (stage), l->data);
+  g_clear_pointer (&monitor_src->watches, g_list_free);
+
+  switch (meta_screen_cast_stream_get_cursor_mode (stream))
+    {
+    case META_SCREEN_CAST_CURSOR_MODE_METADATA:
+    case META_SCREEN_CAST_CURSOR_MODE_HIDDEN:
+      add_view_watches (monitor_src,
+                        META_STAGE_WATCH_BEFORE_PAINT,
+                        before_stage_painted);
+      add_view_watches (monitor_src,
+                        META_STAGE_WATCH_AFTER_ACTOR_PAINT,
+                        stage_painted);
+      break;
+    case META_SCREEN_CAST_CURSOR_MODE_EMBEDDED:
+      add_view_watches (monitor_src,
+                        META_STAGE_WATCH_AFTER_PAINT,
+                        stage_painted);
+      break;
+    }
+}
+
+static void
+on_monitors_changed (MetaMonitorManager             *monitor_manager,
+                     MetaScreenCastMonitorStreamSrc *monitor_src)
+{
+  reattach_watches (monitor_src);
+}
+
 static void
 meta_screen_cast_monitor_stream_src_enable (MetaScreenCastStreamSrc *src)
 {
   MetaScreenCastMonitorStreamSrc *monitor_src =
     META_SCREEN_CAST_MONITOR_STREAM_SRC (src);
   MetaBackend *backend = get_backend (monitor_src);
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
   MetaCursorTracker *cursor_tracker = meta_backend_get_cursor_tracker (backend);
-  ClutterStage *stage;
   MetaScreenCastStream *stream;
 
   stream = meta_screen_cast_stream_src_get_stream (src);
-  stage = get_stage (monitor_src);
 
   switch (meta_screen_cast_stream_get_cursor_mode (stream))
     {
@@ -348,25 +389,21 @@ meta_screen_cast_monitor_stream_src_enable (MetaScreenCastStreamSrc *src)
                                 G_CALLBACK (cursor_changed),
                                 monitor_src);
       meta_cursor_tracker_track_position (cursor_tracker);
-      G_GNUC_FALLTHROUGH;
+      break;
     case META_SCREEN_CAST_CURSOR_MODE_HIDDEN:
-      add_view_watches (monitor_src,
-                        META_STAGE_WATCH_BEFORE_PAINT,
-                        before_stage_painted);
-      add_view_watches (monitor_src,
-                        META_STAGE_WATCH_AFTER_ACTOR_PAINT,
-                        stage_painted);
       break;
     case META_SCREEN_CAST_CURSOR_MODE_EMBEDDED:
       inhibit_hw_cursor (monitor_src);
       meta_cursor_tracker_track_position (cursor_tracker);
-      add_view_watches (monitor_src,
-                        META_STAGE_WATCH_AFTER_PAINT,
-                        stage_painted);
       break;
     }
 
-  clutter_actor_queue_redraw (CLUTTER_ACTOR (stage));
+  reattach_watches (monitor_src);
+  g_signal_connect_object (monitor_manager, "monitors-changed-internal",
+                           G_CALLBACK (on_monitors_changed),
+                           monitor_src, 0);
+
+  clutter_actor_queue_redraw (CLUTTER_ACTOR (get_stage (monitor_src)));
 }
 
 static void