Request: Support font feature: frac

[AndydeCleyre]

Hello, and thanks!

While some font features are already supported, one that is not yet is frac, for displaying fractions in essentially NUMBER/NUMBER instances.

In Wezterm, with

harfbuzz_features = {
  "clig",
  "frac",
  "kern",
  "liga",
  "thnd",
  "txtr",
},

here's the comparison, with Rio on top:

[raphamorim]

Thanks for the issue @AndydeCleyre!

Are those features specifically for harfbuzz (haven't checked yet in harfbuzz)? If yes, then I think will need to thinking in port those to https://github.com/dfrg/swash (that's what rio uses under the hood)

[AndydeCleyre]

frac is part of the OpenType spec, and can be found listed here on Wikipedia and here on the Microsoft OpenType docs. I do see it listed here in the swash source, but I don't know how or if it's fully handled there.

[raphamorim]

aah ok, thanks for clarifying then rio should have support for it already. Could you try using https://rioterm.com/docs/config#fontsfeatures

fonts.features = ["frac"]

[AndydeCleyre]

Yes, thanks, I have that set but there's no visible change.

[raphamorim]

ah i see, i will try to debug later locally after finish #980 PR