title: invert white to black uiimage
body: <p>I have the following UIImage:</p>
  
  <p><img src="https://i.stack.imgur.com/nCpJl.png" alt="enter image description here"></p>
  
  <p>Using Objective-C, I want to be able to invert the white to black and vice versa.</p>
  
  <p>Any help would be appreciated.</p>