Support a conversion precision key

[madig]

One of our designers recently did an outline font and needed to set it to 4000 upM. The conversion precision conversely needed to be higher. This can be done on the command line, but may be better kept inside the UFO?

[anthrotype]

needed to set it to 4000 upM. The conversion precision conversely needed to be higher

as you know, you can specify a conversion error parameter that is relative to the font's UPEM. E.g. 0.001 would mean 1 unit in 1000 UPEM grid, or 4 in 4000 UPEM one.

Regardless, yes we store that parameter in the UFO and/or designspace lib element, under a private cu2qu key.

PR are welcome

[anthrotype]

typo: "yes we can store...", not "yes we store", as we do not right now.