title: Smudge Tool with Opacity
body: <p>I've been trying to create a smudge tool for an image editing application I've been developing. I've found various resources around the web for smudge algorithms, but none of them take into account using a transparent background.</p>
  
  <p>I can't "pick up and paste" a sample of the background because rendering it wouldn't smudge the transparency into the color, as you'd find on smudge tools in GIMP and Photoshop.</p>
  
  <p>How can I smudge both transparency and color?</p>