Adding zoom

To add zooming and panning to the image, we do not need to modify the image_tableau. We create a new tableau which has the image_tableau as a child, and modifies the GL state before passing the draw request event down to the image.

Now we have a zooming image, but the image tableau never needed to know... The maintainer of the image tableau can concentrate on the thorny issues of making glDrawPixels fast on Windows machines, overcoming compiler bugs, and generally writing a high-quality image display routine.