title: Viewport3D Mouse Event doesn't fire when hitting background
body: <p>I set up a Viewport3D with a MouseEventHandler</p>
  
  <pre><code>[...]
  Main3DWindow.MouseUp += new MouseButtonEventHandler(mainViewport_MouseUp);
  [...]
  void mainViewport_MouseUp (object sender, MouseButtonEventArgs e) {
      Point location = e.GetPosition(Main3DWindow);
      ModelVisual3D result = GetHitTestResult(location);
      if (result == null) {
          _CurrentData.Unselect();
          return;
      }
      _CurrentData.SelectItemFromObjectList(result);
  }
  </code></pre>
  
  <p>And it works pretty fine when an object is clicked.
  My expectation was: If no object is clicked (because the user clicked at the background) the result is null. But in fact the mainViewport_MouseUp-method is not even called.</p>
  
  <p>My question: how can i detect clicks on the background of the Viewport3D?</p>