Log Message:
Update xetex to 0.99991

* Drop build date from --version output.
* Drop Mac-specific image loading code, as it makes little sense after we
  dropped xdv2pdf. Supported image formats are the same on all platforms now.
* Fix OpenType math cut-ins support which was not always working properly.
* Avoid Unicode compatibility decomposition fallback for characters not
  supported by the font as it often result in loss of essential characteristics
  (like using regular Latin characters in place of missing math fraktur ones).
  Non compatibility decomposition and composition fallback is still supported.
* For hybrid OpenType/Graphite fonts, we now prefer using OpenType for
  compatibility with pre-0.9999 XeTeX.
* Fix many memory leaks when using AAT fonts on Mac.
* The language font option when loading Graphite fonts in now treated as a
  BCP 47 language tag not and OpenType one.
* Various Unicode fixes for Windows console (thanks to TANAKA Takuji).
* Respect vertical glyph positioning with AAT fonts (fixes, for example,
  rendering of the new Arabic fonts in Mac OS X 10.9).