Only schedule a frame if client has requested a frame callback

When a client hasn't damaged its surface, we only need to schedule an
output frame if the client has requested a frame callback.

sway/desktop/output.c

@@ -681,9 +681,11 @@ static void damage_surface_iterator(struct sway_output *output, struct sway_view
 		wlr_output_damage_add_box(output->damage, &box);
 	}
 
+	if (!wl_list_empty(&surface->current.frame_callback_list)) {
 		output->surface_needs_frame = true;
 		wlr_output_schedule_frame(output->wlr_output);
 	}
+}
 
 void output_damage_surface(struct sway_output *output, double ox, double oy,
 		struct wlr_surface *surface, bool whole) {