Make so that auto white balance is stored per (physical) card, not per signal.

Make so that auto white balance is stored per (physical) card, not per signal.

This fixes an issue where auto white balance would be completely off if
any signals were remapped out of the default. It means you no longer can
duplicate a signal and have different white balance on it, but that seems
narrow enough that one could use manual white balance for that (I can't
really imagine what the use case would be).