title: Convert piece of canvas to blob type
body: <p>I am trying to optimize some code that uses the canvas.drawImage() call. 
  This is a pretty expensive call when called several hundreds or thousands of times. 
  Found out about the canvas.toBlob(callback) method but I want to know if there is a way to get only a section of the canvas. </p>
  
  <p>drawimage() gives much more fine grain control over which sections should be draw.</p>
  
  <pre><code>canvas.drawImage(canvas, sx, sy, sw, sh, dx, dy, dw, dh);
  </code></pre>
  
  <p>Any suggestions / thoughts are welcomed! </p>