I actually did pretty much exactly this for a previous job for testing a rendering engine - we had a 'golden master' set of screenshots, a bunch of code to render those original golden masters, then used perceptual diff (http://pdiff.sourceforge.net) to check against the golden masters.

It was a bit of an experiment and didn't get used that much - though it did come in handy when trying to write UI rendering that worked with XAML