title: "before and after" Interactive image mask
body: <p>I'd like to recreate the before and after image slider recently featured on the guardian.co.uk, using web-kit. </p>
  
  <p><a href="http://www.guardian.co.uk/uk/interactive/2011/aug/09/london-riots-before-after-photographs" rel="nofollow noreferrer">http://www.guardian.co.uk/uk/interactive/2011/aug/09/london-riots-before-after-photographs</a></p>
  
  <p>It's basically two images on top of each other with a vertical slider that follows the mouse, revealing one image on the left and the other on the right.</p>