[gentium] font variants for pdflatex

[Karl Berry]

Hi Thorsten,

(FYI, I only cc'd Bob [now taking him off] because I knew he has made
pdflatex support for many fonts, and thought he might have dealt with
supporting features. As he says, he hasn't worked on Gentium. Although I
wouldn't be surprised if running Gentium through his mill might end up
with just as good of support as what we've got now. Anyway.)

    That should not matter because the featuresets, AIUI, only swap
    glyphs around,

I think features can also affect accent positioning and maybe other
things, but I agree, mostly about swapping glyphs around.

    For the manual process, I'm doing this:

Wow. You are doing far more than anyone else I know has attempted.
It would be fantastic to have a general solution along these lines.

    I tried with Font::TTF but I utterly fail at that,

I am clueless, but maybe Font::TTF is too old for the Gentium ttf's. It
seems the module on CPAN dates from 2016. Since both Font::TTF and
Gentium are released by SIL, could be worth reporting what you're trying
to do + failure to them.  https://github.com/silnrsi/font-ttf

If I were starting down this road (which I'm not, sorry) I would be
tempted to use ttx (fonttools, https://github.com/fonttools/fonttools)
to generate XML for the font and use that to garner the
information. This is what I've used for writing scripts around the
Lucida OTF/TTF fonts -- I never needed "real" XML parsing, just treated
the XML as a text file.

Let me/us know how it goes ... sorry I don't have any real help to offer.
--best, karl.