title: Get canvas from context
body: <p>Is there any way to obtain the canvas that a context is used for?</p>
  
  <p>Why I'm asking is because I'm creating a prototype function for <code>CanvasRenderingContext2D</code> in which I need the width/height of the canvas element.</p>
  
  <p>E.g.:</p>
  
  <pre><code>var cv = document.getElementById('canvas');
  var ctx = cv.getContext('2d');
  // Using only 'ctx', how to get 'cv'?
  </code></pre>