What's the right tolerance for dropping an oncurve

fontmake, and soon fontc - https://github.com/googlefonts/fontations/issues/640 - strive to only drop oncurve when the implied value is *exactly* the same. This means we don't drop an oncurve that would move say 1e-6 font units if it took the implicit value.

If we were willing to drop points that are _very close_ when they take the implied value we could save some bytes. This produces two questions:

1. Do we save enough bytes to care? - if not we can abandon the idea
1. How much shift is OK? Is 1e-6 or 1e-3 font units something anyone cares about?