title: wpf3d rectangle hit test
body: <p>I have an application with 3D objects that reside in Viewport3D and I want the user to be able to select them by dragging a rectangle on the screen.</p>
  
  <p>I tried applying GeometryHitTestParameters (with rectangle geometry) on the Viewport3D in order to get the results, but I get an exception telling me that it is unsuppoted with Viewport3D. Only PointHitTestParameters are supported.</p>
  
  <p>Does anybody know any elegant way to do it, except calculating it myself (for example - projecting all 3D objects to 2D and doing manual geometry intersections with a rectangle)?</p>