wayland: commit scene after dispatch to fix one-frame-behind rendering

Client surface commits arrive during wl_event_loop_dispatch but may be
processed after the frame event has already rendered the old state. Add
an additional wlr_scene_output_commit after dispatch to immediately
pick up any new client content, fixing the one-interaction-behind
visual update issue in WINGs clients like WPrefs.

src/backend/wayland/wl_backend.c

@@ -3679,6 +3679,11 @@ wl_event_loop_run(void)
                 wlr_output_schedule_frame(wl_state.output);
             wl_event_loop_dispatch(loop, 16);
 
+            /* Commit scene after dispatch to pick up any client surface
+             * updates that arrived during this iteration. */
+            if (wl_state.scene_output && wl_state.allocator)
+                wlr_scene_output_commit(wl_state.scene_output, NULL);
+
             /* Check inotify for defaults database changes */
 #ifdef HAVE_INOTIFY
             if (w_global.inotify.fd_event_queue >= 0) {