Avoid recreating the same variation model over and over again

As noted in https://github.com/googlefonts/fontc/pull/403/files#r1302480747, I would imagine that creating a new variation model for every glyph repeats a reasonable amount of work.

I think perhaps an interface to variation modelS where you ask for a model for a set of locations, getting back a possibly shared reference, might result in a simple fix for this and also work well for kerning (#407)