scene: add wlr_scene_get_scene_output

This allows getting a wlr_scene_output from a wlr_output. Since an
output can only be added once to a scene-graph there's no ambiguity.

This is useful for compositors using wlr_scene_attach_output_layout:
the output layout integration automatically creates a scene-graph
output for each wlr_output added to the layout.

include/wlr/types/wlr_scene.h

@@ -282,6 +282,13 @@ bool wlr_scene_output_commit(struct wlr_scene_output *scene_output);
  */
 void wlr_scene_output_for_each_surface(struct wlr_scene_output *scene_output,
 	wlr_surface_iterator_func_t iterator, void *user_data);
+/**
+ * Get a scene-graph output from a wlr_output.
+ *
+ * If the output hasn't been added to the scene-graph, returns NULL.
+ */
+struct wlr_scene_output *wlr_scene_get_scene_output(struct wlr_scene *scene,
+	struct wlr_output *output);
 
 /**
  * Attach an output layout to a scene.

types/scene/wlr_scene.c

@@ -828,6 +828,18 @@ void wlr_scene_output_destroy(struct wlr_scene_output *scene_output) {
 	free(scene_output);
 }
 
+struct wlr_scene_output *wlr_scene_get_scene_output(struct wlr_scene *scene,
+		struct wlr_output *output) {
+	struct wlr_addon *addon =
+		wlr_addon_find(&output->addons, scene, &output_addon_impl);
+	if (addon == NULL) {
+		return NULL;
+	}
+	struct wlr_scene_output *scene_output =
+		wl_container_of(addon, scene_output, addon);
+	return scene_output;
+}
+
 void wlr_scene_output_set_position(struct wlr_scene_output *scene_output,
 		int lx, int ly) {
 	if (scene_output->x == lx && scene_output->y == ly) {