title: Making a SKScene's background transparent not working... is this a bug?
body: <p>Is there a way to make a SKScene's background transparent and present that scene over another one seeing thru the transparency.</p>
  
  <p>The idea is to have the background of the presented scene like this:</p>
  
  <pre><code>self.backgroundColor = [SKColor colorWithRed:0.0f green:0.0f blue:0.0f alpha:0.5f];
  </code></pre>
  
  <p>what would allow to see the scene behind darken. But doing this is not working. Background is presented completely opaque.</p>
  
  <p>Is there a way to do that?</p>