What's Changed

• General shaping and subsetting speedups.
• Fix in Graphite shaping backend when glyph advances became negative.
• Subsetting improvements, pruning empty mark-attachment lookups.
• Don't use the macro name _S, which is reserved by system liberaries.
• Build fixes and speedup.
• Add a kbts shaping backend that calls into the kb_text_shape
  single-header shaping library. This is purely for testing and
  performance evaluation and we do NOT recommend using it for any
  other purposes.