title: How to flip coordinates when drawing in context?
body: <p>I create a context from an UIImage, and then I draw into it with </p>
  
  <pre><code>CGContextDrawImage(bitmapContext,
                     CGRectMake(0, 0,
                                originalImage.size.width,
                                originalImage.size.height),
                     oImageRef); 
  </code></pre>
  
  <p>the image appears upside-down due to the flipped coordinate system in quartz. How can I fix that?</p>