[subset] During LigatureSubstFormat1 subsetting always place Coverage last.

In Windows 7 on Chrome if the coverage table comes before any of the LigatureSet or Ligature subtables the font won't load. This changes the packing order to always place the Coverage table last. Virtual links are used to ensure the repacker maintains the desired ordering.

Coincidentally fontTools also does the same thing (https://github.com/fonttools/fonttools/blob/a3f988fbf621142d7d9b3e68a69725b88f358869/Lib/fontTools/ttLib/tables/otTables.py#L1137) to reduce overflows during packing.
44 files changed