title: UWP CanvasRenderTarget giving corrupt image
body: <p>I'm doing a "hello world" type test with win2d. 
  In my canvas draw event, this gives me a nice round circle as I expect:</p>
  
  <pre><code>args.DrawingSession.FillCircle(100, 100, 50, Windows.UI.Colors.Red)
  </code></pre>
  
  <p>But I would like to draw "off screen", so I tried this:</p>
  
  <pre><code>    Dim dev As CanvasDevice = sender.Device
      Dim crt As New CanvasRenderTarget(dev, CSng(sender.ActualWidth), CSng(sender.ActualHeight), sender.Dpi)
      Dim ds As CanvasDrawingSession = crt.CreateDrawingSession
      ds.FillCircle(100, 100, 50, Windows.UI.Colors.Red)
  
      args.DrawingSession.DrawImage(crt)
  </code></pre>
  
  <p>This sort of works but as I resize the page to force redraws, I see all sorts of strange corrupted images, looks like tearing, uninitialsed memory, etc. </p>
  
  <p>What is going on and how do I fix this?
  Many thanks.</p>