title: masking UIView based on CGRect
body: <p>Look at the image please : 
  <img src="https://i.stack.imgur.com/enycx.png" alt="enter image description here"></p>
  
  <p>I have UIView and CGRect. I have to check where CGRect overlays UIView and then set the mask on the area which is "opposed" to intersection. So it's blue part in the image. How to achieve that? 
  I know there's CGContextClipToMask but I have no idea how to use that - could someone help me with example code? Is CGContextClipToMask good direction? </p>
  
  <p>Thanks! </p>