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
0c2eed533e