title: How to use compositingFilter in CALayer?
body: <p>I just noticed that a <strong>CALayer</strong> can have a  <strong>compositingFilter</strong> as a property.</p>
  
  <p>But a <strong>compositingFilter</strong> requires an  <strong>inputImage</strong> and an <strong>inputBackgroundImage</strong> as its inputs. Then the filter provide output image in a method.</p>
  
  <p>So how should I use this filter as a property of <strong>CALayer</strong>? What are the inputs and output?</p>