title: iOS invert mask in drawRect
body: <p>With the code below, I am successfully masking part of my drawing, but it's the inverse of what I want masked. This masks the inner portion of the drawing, where I would like to mask the outer portion. Is there a simple way to invert this mask?</p>
  
  <p><code>myPath</code> below is a <code>UIBezierPath</code>.</p>
  
  <pre><code>CAShapeLayer *maskLayer = [[CAShapeLayer alloc] init];
  CGMutablePathRef maskPath = CGPathCreateMutable();
  CGPathAddPath(maskPath, nil, myPath.CGPath);
  [maskLayer setPath:maskPath];
  CGPathRelease(maskPath);
  self.layer.mask = maskLayer;
  </code></pre>