[XeTeX] Support of variable OpenType fonts

[Karl Berry]

Hi Bruno and all,

    Does XeTeX support variable OpenType fonts, or is the addition of their
    support planned at some point?

Jonathan or Arthur or anyone can correct me, but so far as I have
observed, no one has been working on such new xetex developments for
years now.

If there is anyone out there with sufficient time, interest, and skill,
a volunteer would surely be welcome. If a small grant would be any
enticement for this particular (or other) task(s), the TeX development
fund (tug.org/tc/devfund) is available.

    includes the STIX Two fonts in a slightly more recent version than TeX
    Live (2.13 b171 for macOS vs 2.12 b168 for TL).

TL follows whatever is uploaded to CTAN. Maybe you could write the stix2
maintainers about keeping CTAN up to date?

    the text fonts are in variable OpenType format,

Seems like a poor decision wrt backward compatibility, but I'm sure
whoever changed it had good reasons.

(Especially since, I heard vaguely, it is XeTeX which AMS uses
occasionally, never LuaTeX, and AMS is one of the primary Stix2
sponsors. Anyway.)

            \fonttest{[STIXTwoText-Italic.ttf]:axis={weight=medium}}

    for macOS (for some reason instance= doesn't work in this latter case).

Maybe something to report to luaotfload, except they surely won't be
able to do anything without the actual font.

Thanks for the report. --karl.