scene: add a function to get a node from a surface

[emersion]

struct wlr_scene_surface *wlr_scene_surface_from_surface(struct wlr_scene *scene,
	struct wlr_surface *surface);

Problem: this makes it impossible for compositors to add the same wlr_surface to a scene-graph multiple times.

---

wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3310