scene/output-layout: improve ownership logic

This commit ensures that outputs that weren't created by the output
layout helper aren't destroyed on the output layout change.

Consider the following piece of logic:

// struct wlr_output *o1, *o2;
// struct wlr_scene *scene;
// struct wlr_output_layout *layout;
wlr_scene_attach_output_layout(scene, layout);
wlr_output_layout_add_auto(layout, o1);
struct wlr_scene_output *so2 = wlr_scene_output_create(scene, o2);
wlr_output_layout_move(layout, o1, 100, 200);
// so2 is invalid now

1 files changed, 4 insertions, 2 deletions

diff --git a/include/wlr/types/wlr_scene.h b/include/wlr/types/wlr_scene.h
index 4b8ea517..49804d83 100644
--- a/include/wlr/types/wlr_scene.h
+++ b/include/wlr/types/wlr_scene.h
@@ -406,8 +406,10 @@ struct wlr_scene_output *wlr_scene_get_scene_output(struct wlr_scene *scene,
 /**
  * Attach an output layout to a scene.
  *
- * Outputs in the output layout are automatically added to the scene. Any
- * change to the output layout is mirrored to the scene-graph outputs.
+ * Adding, removing, or repositioning an output in the output layout
+ * will respectively add, remove or reposition a corresponding
+ * scene-graph output. When the output layout is destroyed, scene-graph
+ * outputs which were created by this helper will be destroyed.
  */
 bool wlr_scene_attach_output_layout(struct wlr_scene *scene,
 	struct wlr_output_layout *output_layout);