Hi,
Theoretically, keystoning can be a function of a video processor. It would be digital keystoning, similiar to what's found in digital projectors.
It would degrade picture quality relative to an unkeystoned image, although it might be better than having no scaler at all (depending on how good the keystoning interpolation is).
It's definitely not recommended either way to use digital keystone of any kind, if you can avoid it.