Fall back to gvar for metric deltas

[valadaptive]

Like with googlefonts/fontations#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?)

[xorgy]

@valadaptive

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

If somebody is willing to do the work, anything in Swash that has an equivalent in Skrifa should be replaced with that equivalent. Swash was the proving ground for significant parts of Skrifa, but Skrifa should be considered to supersede Swash's versions of these things.

[xorgy]

I noticed you have a skrifafy branch which presumably is for this purpose. :+ )