Per-script/language vertical metrics

[behdad]

The BASE table in OpenType has a MinMax extents table per script/language:

https://learn.microsoft.com/en-us/typography/opentype/spec/base#the-minmax-table-and-featminmaxrecord

which is AFAIK under-specified and unimplemented by clients. Plan is to revive and use that as vertical ascent/descent. So, to that extent, the spec needs clarification but the file format needs no change. The implementations (eg. HarfBuzz) would need API to expose that. We have an issue for that:

https://github.com/harfbuzz/harfbuzz/issues/3290

[behdad]

HarfBuzz patch in https://github.com/harfbuzz/harfbuzz/pull/4237