title: How do I get the frame of visible content from SKCropNode?
body: <p>It appears that, in SpriteKit, when I use a mask in a SKCropNode to hide some content, it fails to change the frame calculated by <code>calculateAccumulatedFrame</code>. I'm wondering if there's any way to calculate the visible frame.</p>
  
  <p>A quick example:</p>
  
  <pre><code>import SpriteKit
  
  let par = SKCropNode()
  
  let bigShape = SKShapeNode(rect: CGRect(x: 0, y: 0, width: 100, height: 100))
  bigShape.fillColor = UIColor.redColor()
  bigShape.strokeColor = UIColor.clearColor()
  par.addChild(bigShape)
  
  let smallShape = SKShapeNode(rect: CGRect(x: 0, y: 0, width: 20, height: 20))
  smallShape.fillColor = UIColor.greenColor()
  smallShape.strokeColor = UIColor.clearColor()
  par.maskNode = smallShape
  
  par.calculateAccumulatedFrame() // returns (x=0, y=0, width=100, height=100)
  </code></pre>
  
  <p>I expected <code>par.calculateAccumulatedFrame()</code> to return <code>(x=0, y=0, width=20, height=20)</code> based on the crop node mask.</p>
  
  <p>I thought maybe I could code the function myself as an extension that basically reimplements <code>calculateAccumulatedFrame</code> with support for checking for SKCropNodes and their masks, but it occurred to me that I would need to consider the alpha of that mask to determine if there's actual content that grows the frame. Sounds difficult.</p>
  
  <p>Is there an easy way to calculate this?</p>