Kerning exceptions should fall back to class kerning in locations where they are undefined

[GaetanBaehr]

I'm looking at a project designspace + ufos, if I open the designspace with Fontra Pak on some sources I doesn't see any kerning in Fontra. But if I open separatly the .ufos with Fontra Pak then I see correctly the kerning.

Below the preview of a source from the designspace in Fontra and on the bottom the preview from RoboFont's space center

Below the preview of the ufo of the same source in Fontra

[justvanrossum]

Hm weird! I can reproduce (I have these fonts). I'll have a look.

[justvanrossum]

The problem is:

• Only some UFO sources have A V as an individual pair (kerning exception)
• This overrides the @A V pair for all sources

I'm not sure how fontmake makes sense of this, but Fontra clearly gets confused. I'm not sure how to resolve this, given how Fontra treats kerning as "one pair for all sources". It currently cannot deal with kerning exceptions per source.

[justvanrossum]

I have an idea how I could fix this, but it's not quite trivial.