The associated `ViewportTexture`s will update the `viewport_path`
in time when the `Viewport`'s nodepath is changed (caused by renaming
the node names or moving in the SceneTree dock).
If the target `Viewport` is changed by resetting the `viewport_path`,
the `ViewportTexture`s will be re-setup and emit `changed` signal in
time.
(cherry picked from commit af58f1e854)