title: Cut a UIImage into a circle
body: <p>I want to cut a <code>UIImage</code> into a circle so that I can then use it as an annotation. Every answer on this site that I've found describes creating an <code>UIImageView</code>, then modifying that and displaying it, but you cant set the image of an annotation to an <code>UIImageView</code>, only a <code>UIImage</code>.
  How should I go about this?</p>