[subset] Support GSUB/GPOS.FeatureVariations
[subset] Support GSUB/GPOS.FeatureVariations

We should add a test using the font at:
https://github.com/behdad/harfbuzz/issues/521

Test with:
$ fonttools subset avar_rvrn.ttf --text=r

googlefonts/fonttools,fonttools/fonttools