Support for "vrt2" and other vertical writing features

[fuma-nama]

I'm currently trying to parse and render Japanese fonts in vertical writing mode. While it's clear that only ligature features like "liga" and "rlig" are supported, most of the features are not supported. And vmtx table is not yet implemented according to the roadmap, would that be considered in the near future?

[ILOVEPIE]

This is of interest to me too, there's a few PRs that need to be merged first but once those are merged, we can look into this. We'd greatly appreciate any contribution you have to offer @fuma-nama, but if you can't get to it, I'll look into it later.