title: ILShapes not drawn when added as a children to ILScreenObject
body: <p>I am generating image plot by setting the surface plot in 2d mode. I am trying to draw the overlay crosshair as an ILScreenObject and move it to jog/navigate
  through the surface points along the wireframe.</p>
  
  <p>But any shapes I add to the ILScreenObject are not visible other than the ILLabel?
  I am using the following code.</p>
  
  <pre><code>        ILArray&lt;float&gt; specialData = ILSpecialData.sincf(20, 30);
          ILSurface surface = new ILSurface(specialData);
          surface.MouseClick += surface_MouseClick;
          ILPlotCube cube = new ILPlotCube();
          cube.Add(surface);
  
          //construct ILScreenobject
          ILScreenObject screenObj = new ILScreenObject();
          screenObj.Children.Add(new ILLabel("test label"));
          screenObj.Children.Add(Shapes.RectangleFilled);
  
          ilPanel1.Scene.Add(cube);
          ilPanel1.Scene.Camera.Add(screenObj);
          ilPanel1.Refresh();
  </code></pre>
  
  <p>All I could see with this code is a label centered on the image plot. Could anyone suggest me any possible solution for this? Thanks.</p>