title: CIDepthBlurEffect: setting inputFocusRect
body: <p>I can't manage to find the correct way to set the "inputFocusRect" value from a touch point. 
  I'm grabbing the touch point using</p>
  
  <pre><code>@IBAction func imageTapped(_ sender: UITapGestureRecognizer) {
      var touchPoint = sender.location(in: self.imageView)
      imageTapCoordinates = CIVector(cgPoint: touchPoint)
  </code></pre>
  
  <p>I pass that to the CIFilter in a separate blur function. </p>
  
  <pre><code>filter?.setValue(imageTapCoordinates, forKey: "inputFocusRect")
  </code></pre>
  
  <p>However it seems like the coordinate systems don't match up! What's the best way of determining the "inputFocusRect"? Apple doesn't have much documentation on this on their site...</p>
  
  <p>Thanks!</p>