I doubt they're doing that, but it can be done realtime. Here's an example that runs in your browser, just move your mouse around and her eyes track it. http://alteredqualia.com/xg/examples/gaze_dolores.html

[Edit: Disclaimer, I work at MS but not on Surface. I also don't know if there is post-processing applied to the video I linked, so maybe it can't be done realtime.]