private void OnManipulationDelta(ManipulationDeltaRoutedEventArgs args)
 {
     if (_currentStroke != null)
     {
         _currentStroke.AddPoint(args.Position);
     }
     if (_clipboardBorder != null)
     {
         _clipboardBorder.AddPoint(args.Position);
     }
     _canvasControl.Invalidate();
 }