toplevel-capture: work around stalls for captured windows in the capture scene

This is part of the missing frame event workaround, this one handles
stalls of the capture side when changing the visibility of the captured
surface on the usual output.

include/view.h

@@ -180,6 +180,7 @@ struct view {
 	struct {
 		struct wlr_scene *scene;
 		struct wlr_ext_image_capture_source_v1 *source;
+		struct wl_listener on_capture_frame;
 		struct wl_listener on_capture_source_destroy;
 	} capture;