[GDEF/GSUB/GPOS] Coverage / ClassDef tables >64k expansion

[behdad]

Two new formats each:

struct CoverageFormat3 {
  uint16 format; // == 3
  Array16Of<GlyphID24> glyphs;
};

struct CoverageFormat4 {
  uint16 format; // == 4
  Array16Of<Range24Record> ranges;
};

struct ClassDefFormat3 {
  uint16 format; // == 3
  GlyphID24 startGlyphID;
  Array24Of<uint16> classes;
};

struct ClassDefFormat4 {
  uint16 format; // == 4
  Array24Of<Range24Record> ranges;
};

struct Range24Record {
  GlyphID24 startGlyphID;
  GlyphID24 endGlyphID;
  uint16 value;
};

[behdad]

Speced: https://github.com/harfbuzz/boring-expansion-spec/blob/main/beyond-64k.md
Implemented in HarfBuzz.