Actually not really related to this PR, but is there a reason not to use the std::visit(variant, overloaded {...}) pattern? (see example 4 of https://en.cppreference.com/w/cpp/utility/variant/visit2.html, or grep for overloaded in the mplcairo codebase)

I've not tried std::visit, but at least for std::get_if, one reason why we stuck with it instead of something else is that it wouldn't compile on macOS with the current deployment target we have. It's possible that it's fine now, but I don't believe we've bumped the target in a while.

In the long term it would be nice if mplcairo could directly call this (instead of having to reimplement the whole thing itself as well); but I suspect the issue is the fact that this has to expose FT2Font objects and mplcairo would thus need to also use the same ft2Library instance as matplotlib, which... well I don't really know if that's something matplotlib wants to expose publically. (I'm mostly just thinking aloud here.)
(mplcairo could also just grab the filename of the FT2Font objects and reopen them, which is not too bad with caching, but I'm not sure this would work with variable fonts at least?)

There would need to be some way to share the FreeType library itself, I think. One issue I had initially was between sharing the internal FreeType structures with another when I accidentally had libraqm link to an external one. I'm not sure how easy this would be, unless we moved to some common provider.