title: How to draw a drop-shadow on a transparent rectangle?
body: <p>When I add a dropshadow bitmap effect to a rectangle, the dropshadow takes into account the transparency of the rectangle (makes sense). Is there a way to render a dropshadow on a transparent rectangle 'as if' the rectangle were opaque? ie what would appear is a rectangle-shaped 'hole', with a dropshadow.</p>
  
  <p>Here is the XAML for a transparent rectangle with a dropshadow - nothing is displayed:</p>
  
  <pre><code>&lt;Rectangle Fill="Transparent" Margin="10" Width="100" Height="100"&gt;
    &lt;Rectangle.BitmapEffect&gt;
      &lt;DropShadowBitmapEffect/&gt;
    &lt;/Rectangle.BitmapEffect&gt;
  &lt;/Rectangle&gt;
  </code></pre>