OTF feature tags

[emilk]

OTF files have feature flags, like tnum.

It would be wonderful to be able to enable such flags in ab-glyph

[alexheretic]

Yet it would be nice to have. The first step would be figuring out exactly how they work using ttf-parser crate.

[simoncozens]

You're going to need a shaping engine like harfbuzz_rs/rustybuzz/allsorts to execute the substitution and positioning rules in the GSUB/GPOS tables stored with each feature. It does get very complicated (particularly with lookup flags and especially with complex scripts), so I would advise against trying to implement shaping in this crate / with ttf-parser.