[GSUB/GPOS] Main struct >64k expansion

[behdad]

Currently the tables cannot support more than ~3k lookups before the LookupList offsets themselves overflow. I hit this limit when trying to merge all 180 Noto Sans fonts. To address this, the main GSUB/GPOS structs need to be bumped to version 0x00020000.

struct GSUBGPOSVersion2 {
  Version version; // 0x00020000
  Offset24To<ScriptList> scripts;
  Offset24To<FeatureList> features;
  Offset24To<LookupList24> lookups;
  Offset32To<FeatureVariations> featureVars;
};

Note that the last item is a 32bit offset, for compatibility with version 0x00010001 tables.

were LookupList24 is a list16 of offset24 to Lookup structures:

using LookupList24 = List16OfOffsetTo<Lookup, uint24>;

[behdad]

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