Try baking soda and white vinegar. Add vinegar to the baking soda until it's a paste (not quite liquid). Lightly rub it on the glass, then let it sit for 10 minutes or so. Then rub it into the glass with some elbow grease, then wipe clean with damp cloth.
May have to try a few times. I've had some good luck with it before, but I haven't run into anything too tough yet. Not sure if it'll be strong enough or not, might be worth a shot. I was going to try the Mr. clean magic erasers one day too but haven't gotten around to it yet.