Clear the previously set state when configuring for a new scene root node

Saving a subscene causes the main scene to be re-instantiated. And the resource
instance in the main scene will be reused when the main scene is re-instantiated.
So for resources with `resource_local_to_scene` enabled, resetting state may be
necessary (at least for `ViewportTexture`).

scene/main/viewport.h

@@ -70,6 +70,8 @@ class ViewportTexture : public Texture2D {
 protected:
 	static void _bind_methods();
 
+	virtual void reset_local_to_scene() override;
+
 public:
 	void set_viewport_path_in_scene(const NodePath &p_path);
 	NodePath get_viewport_path_in_scene() const;