Event callback for a world transform-plane change event. This only fires when the mapping between 2D physics space and 3D world space actually changes. A world using PhysicsWorld.TransformPlane.Custom also fires this when PhysicsWorld.transformPlaneCustom changes, reporting PhysicsWorld.TransformPlane.Custom as both the old and the new plane.
Do not treat a difference between the old and new plane as the test for whether anything changed, because a change to the custom transformation alone reports the same plane on both sides.