[HarfBuzz] Reading a GSUB subtable

[Owen Lamb]

Hello,

I tried sending this once, but it apparently went to moderator approval
because I wasn't a member of the list. It's been a few days without
response, so I decided to sign up for the list and try again. Hopefully
it'll make it through this time! My original message is as follows:

I'm working on a GSoC project for the LilyPond music engraving program,
changing the music notation font standard to SMuFL, which ideally supports
OpenType features, i.e. ligatures and stylistic alternates. For the most
part, we've been using the FreeType library, but since FT doesn't support
OT features, I've begun to look elsewhere. HarfBuzz seems like a good
choice, as we already use Pango elsewhere in the program, so we already
have a dependency on it.

However, I'm having trouble navigating the documentation, in particular how
to read the GSUB table's entire 'salt' lookup subtable at once (and others,
as needed), as an easy-to-use format, e.g. std::map. Is there a way to take
a font face loaded through FT and extract feature subtables from it in this
way?

Thanks in advance,
Owen Lamb
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