title: FabricJS: bug with opacity of circles in a group?
body: <p>When I add a circle with an opacity lower than 1 to a group, its opacity becomes actually lower than the specified value. This does not happen if I don't specify opacity (i.e., opacity = 1). It also doesn't happen with a rectangle.</p>
  
  <p>Here is the code to reproduce this issue:</p>
  
  <h1>HTML</h1>
  
  <pre><code>&lt;canvas id="stage" width="400" height="300"&gt;
  </code></pre>
  
  <h1>JavaScript</h1>
  
  <pre><code>var OPACITY = 0.65;
  var FILL = '#fff';
  
  var canvas = new fabric.Canvas('stage', {
      backgroundColor: '#222'
  });
  
  /**
   * Rectangles
   * both appear to have the same color
   */
  var rect1 = new fabric.Rect({
      width: 40,
      height: 40,
      fill: FILL,
      opacity: OPACITY,
      left: 60,
      top: 60
  });
  canvas.add(rect1);
  
  var rect2 = new fabric.Rect({
      width: 40,
      height: 40,
      fill: FILL
      opacity: OPACITY,
  });
  var rect2Group = new fabric.Group([rect2], {
      left: 120,
      top: 60
  });
  canvas.add(rect2Group);
  
  /**
   * Circles
   * the second circle is darker
   */
  var circle1 = new fabric.Circle({
      radius: 20,
      fill: FILL,
      opacity: OPACITY,
      left: 60,
      top: 120
  });
  canvas.add(circle1);
  
  var circle2 = new fabric.Circle({
      radius: 20,
      fill: FILL,
      opacity: OPACITY,
  });
  var circle2Group = new fabric.Group([circle2], {
      left: 120,
      top: 120
  });
  canvas.add(circle2Group);
  </code></pre>
  
  <p>Here is the <a href="http://jsfiddle.net/pYhRV/" rel="nofollow">JSFiddle</a>.</p>
  
  <p>If you run it, you can see that the second circle is darker than the first one, meaning that its opacity is lower.</p>
  
  <p>Am I doing something wrong, or is this a bug? (Could be reproduced in 1.2.0 and 1.3.0.)</p>