title: Create a CGContextRef for existing CGImageRef
body: <p>Suppose I have an existing CGImageRef. I would like to create a CGContextRef for that existing  image and draw something in it. </p>
  
  <p>Considering that the CGImageRef has an underlying data array in memory, my guess is that it should be possible to create a new context using <code>CGBitmapContextCreateWithData</code> and pass as data array the underlying memory data from CGImageRef. </p>
  
  <p>Unfortunately, I don't know how to get the data array from a CGImageRef (might not even be possible).</p>
  
  <p>Please note that I prefer not to create a new context, draw the old image and then the new content. </p>