scene: ignore duplicate wlr_scene_output_layout_add_output() calls

An assert was added in [1] to avoid tracking the same output multiple
times. However, this is cumbersome for compositors [2]: they need to
add a special check for this. Additionally, this is inconsistent with
wlr_output_layout_add().

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4380
[2]: https://github.com/cage-kiosk/cage/pull/315

types/scene/output_layout.c

@@ -75,7 +75,9 @@ void wlr_scene_output_layout_add_output(struct wlr_scene_output_layout *sol,
 
 	struct wlr_scene_output_layout_output *solo;
 	wl_list_for_each(solo, &sol->outputs, link) {
-		assert(solo->scene_output != so);
+		if (solo->scene_output == so) {
+			return;
+		}
 	}
 
 	solo = calloc(1, sizeof(*solo));