title: TransparencyKey leaves pixels on rounded corner background images
body: <p>I'm trying to use a background image with rounded corners against a 'lime' background colour with the transparencykey set to 'lime', it mostly has the desired effect of just showing all the form controls on the background image but the lime background is not removed from the transparency key for 1px around the rounded edges:</p>
  
  <p><a href="https://i.stack.imgur.com/K3aCp.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/K3aCp.png" alt="1px ring left on all 4 corners"></a></p>
  
  <p>Does anyone know how i can prevent this happening?</p>