Fixed Timestep Interpolation (2D)

Adds fixed timestep interpolation to the rendering server (2D only).
Switchable on and off with a project setting (default is off).

Co-authored-by: lawnjelly <lawnjelly@gmail.com>

scene/main/canvas_item.h

@@ -140,6 +140,8 @@ private:
 
 	void _notify_transform(CanvasItem *p_node);
 
+	virtual void _physics_interpolated_changed() override;
+
 	static CanvasItem *current_item_drawn;
 	friend class Viewport;
 	void _refresh_texture_repeat_cache() const;