fix black screen on QEMU modeset/resize

When running dwl inside a QEMU VM and toggling fullscreen in and out,
the output can stay as a black screen until a new client (such as foot)
forces a redraw. Calling wlr_output_schedule_frame() ensures the frame
loop stays active and resolves the issue.

dwl.c

@@ -2430,6 +2430,7 @@ rendermon(struct wl_listener *listener, void *data)
 
 	wlr_scene_output_commit(m->scene_output, NULL);
 	clock_gettime(CLOCK_MONOTONIC, &now);
+	wlr_output_schedule_frame(m->wlr_output);
 	wlr_scene_output_send_frame_done(m->scene_output, &now);
 }
 
@@ -2459,6 +2460,7 @@ requestmonstate(struct wl_listener *listener, void *data)
 {
 	struct wlr_output_event_request_state *event = data;
 	wlr_output_commit_state(event->output, event->state);
+	wlr_output_schedule_frame(event->output);
 	updatemons(NULL, NULL);
 }