screen-cast/src: Allow dropping cursor-frame if nothing changed

Allow a screen cast stream source to say that nothing changed in terms
of cursor metadata, and treat this together with a cursor-only frame as
we not recording anything.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3803>

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

@@ -776,6 +776,8 @@ meta_screen_cast_stream_src_maybe_record_frame_with_timestamp (MetaScreenCastStr
 {
   MetaScreenCastStreamSrcPrivate *priv =
     meta_screen_cast_stream_src_get_instance_private (src);
+  MetaScreenCastStreamSrcClass *klass =
+    META_SCREEN_CAST_STREAM_SRC_GET_CLASS (src);
   MetaScreenCastRecordResult record_result =
     META_SCREEN_CAST_RECORD_RESULT_RECORDED_NOTHING;
   MtkRectangle crop_rect;
@@ -787,6 +789,15 @@ meta_screen_cast_stream_src_maybe_record_frame_with_timestamp (MetaScreenCastStr
   COGL_TRACE_BEGIN_SCOPED (MaybeRecordFrame,
                            "Meta::ScreenCastStreamSrc::maybe_record_frame_with_timestamp()");
 
+  if ((flags & META_SCREEN_CAST_RECORD_FLAG_CURSOR_ONLY) &&
+      klass->is_cursor_metadata_valid &&
+      klass->is_cursor_metadata_valid (src))
+    {
+      meta_topic (META_DEBUG_SCREEN_CAST,
+                  "Dropping cursor-only frame as the cursor didn't change");
+      return record_result;
+    }
+
   /* Accumulate the damaged region since we might not schedule a frame capture
    * eventually but once we do, we should report all the previous damaged areas.
    */

src/backends/meta-screen-cast-stream-src.h

@@ -85,6 +85,7 @@ struct _MetaScreenCastStreamSrcClass
 
   gboolean (* get_videocrop) (MetaScreenCastStreamSrc *src,
                               MtkRectangle            *crop_rect);
+  gboolean (* is_cursor_metadata_valid) (MetaScreenCastStreamSrc *src);
   void (* set_cursor_metadata) (MetaScreenCastStreamSrc *src,
                                 struct spa_meta_cursor  *spa_meta_cursor);