title: Rounding corner causes top edge to blur
body: <p>I have a UIButton subclass that I am rounding the corners of. Using either the usual <code>cornerRadius</code> property on its layer, or creating a rounded mask and applying that to the layer, I always get the effect shown in the image below (blown up so you can see it clearly). The top pixel is slightly transparent, making the edge look soft. If I remove the rounded corners, the edge goes back to solid (like the bottom edge in the image), so I know it's not just trying to draw the view between pixels.</p>
  
  <p>Any ideas?</p>
  
  <p><img src="https://i.stack.imgur.com/teiv4.png" alt="Corner"></p>