You can get contact cement off of glass by rubbing over it with plain old salad oil and then washing it with soapy water. Since it's only oil and soap, it can't hurt to try at least in a small spot. Something about rubbing the oil into the cement makes it lose it's sticky-ness.
If the surface is fairly smooth and you are going over it with a new pad, it might not be worth worrying about any way. Just a thought.
The fact that this windy world is largely covered in water obviously means that man was meant to sail.