TODO

- Get rid of custom GSUB/GPOS table type enum?
- Test OpenType 1.6 mark filtering sets
- When sanitizing, have to do a second round to make sure no toe-stepping
- HB_OT_LAYOUT_GLYPH_CLASS_BASE_GLYPH vs LookupType::... mess
- Rename LookupFlag::MarkAttachmentType to LookupFlag:IgnoreSpecialMarks
- cmap14
- size_t?