Change the skin (in skinCoord), to follow the animation of joints. Joints that changed are assumed to be already traversed by TX3DNode.Traverse (this prepares field values for AnimateSkinPoint call).
This method is used by TCastleSceneCore. Don't call this yourself.
Returns changed VRML/X3D coordinates field (or Nil if there is no skin). You should call TX3DField.Changed on it (unless you use it from some TCastleSceneCore initialization, when there's no point in notifying about this).