[`cmap`] >64k expansion

[behdad]

Formats 12/13 already support 32bit glyph indices.

Add format16 subtable, which is exactly like format14, except that whereas format14 uses:

struct UVSMapping {
  uint24 unicodeValue;
  uint16 glyphID;
};

the new format16 will use:

struct UVSMapping {
  uint24 unicodeValue;
  uint24 glyphID;
};

(or uint32, if we choose to go that way instead; but I think 24 is more appropriate).

[behdad]

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

[behdad]

NOT implemented in HarfBuzz yet.

[Antecer]

I think we need use offset + index to find glyh, this will significantly compress the length of the cmap table if it is large. just need use uint64 glyphID = (uint64)offset + (uint16)index.