title: Openg GL ES draw offscreen to provide the contents for a CALayer
body: <p>Is it is possible use Open GL ES to draw offscreen to create a CGImageRef to use as content for a CALayer.</p>
  
  <p>I intend to alter the image only once. In detail I'm looking for an efficient way to change only the hue of an image without changing the brightness as well. The other solution might be to create a pixel buffer and to change the data directly but it seems computationally expensive.</p>