Fall back to gvar for metric deltas

[valadaptive]

Like with https://github.com/googlefonts/fontations/pull/497, we were previously not falling back to phantom point deltas if the hvar table was missing. This implements that.

I've used skrifa/read_fonts for this since it was already being used for adjust_axis and means we don't have to reimplement all the parsing here. This does require transmuting the coords from &[i16] to &[F2Dot14], but there are some transmutes like that already in the codebase (e.g. in src/text/compose.rs).

(Speaking of, which parts of this library are you planning to replace with skrifa?)