title: Mac OS X Cocoa, Flipping Y Coordinate in Global Screen Coordinate Space
body: <p>How do I flip the y value of an NSPoint to a flipped coordinate space.</p>
  
  <p>I have two sources of points, both in global screen space, but one is flipped:</p>
  
  <ol>
  <li><p>one from a screen coordinate space with 0,0 = top left.</p></li>
  <li><p>one from a screen coordinate space with 0,0 = bottom left.</p></li>
  </ol>
  
  <p>I need to flip the second one (bottom left) to be in the same space as the first one, top left. How can I do that on an NSPoint or CGPoint?</p>