gvar optimisation breaks variable font rendering in Adobe CC

Earlier today, Kostas Bartsokas tweeted:

I am going crazy with this. Left side: Variable font from Glyphs. Right side: Variable font with fontmake from designspace. Bug appears only on Adobe Apps, thinnest styles (compressed, condensed, narrow). What is happening?

— Kostas Bartsokas (@kostasbartsokas) January 21, 2022

I investigated and it turned out the problem was due to the IUP optimisation in the gvar table. With the IUP optimisation turned off (pass the `--no-optimize-gvar` flag to fontmake), the font works fine. So I conclude that Adobe's current implementation of variable fonts does not correctly handle interpolation of untouched points.

So, three things:

* If you're finding this issue in the future because your font is broken in Adobe, add `--no-optimize-gvar`.
* Should we keep gvar optimisation as the default? I think so; Adobe should fix their bugs.
* Anyone know how best to raise this with Adobe?