Known bugs

- The OT ExtensionSubst and ExtensionPos subtables are not yet
  supported
- The OT FeatureParams subtable is not yet supported
- CFF/CID support fonts is currently limited
- 'kern' subtable format 2 has not been implemented properly
- The code has hardly been tested on 64-bit platforms -- there
  are still some places where it assumes a long is 32 bits.