title: Canvas requestAnimationFrame pause
body: <p>How to pause canvas animation made with requestAnimationFrame ? 
  I start animation like this:</p>
  
  <p>Code:</p>
  
  <pre><code>window.requestAnimFrame = (function() {
      return  window.requestAnimationFrame       ||
              window.webkitRequestAnimationFrame ||
              window.mozRequestAnimationFrame    ||
              function(callback) {
                  window.setTimeout(callback, 1000 / 60);
              };
  })();
  
  function Start() {
      Update();
      requestAnimFrame(Start);
  }
  
  Start();
  </code></pre>
  
  <p>Now I want to add pause option after keydown. Is there any simple way to do it?</p>