[HarfBuzz] Memory leak when destroying hb-ft-fonts after shaping

[moritz blanckenburg]

Here's a short program that recreates the bug:
https://gist.github.com/3229809

Some notes:
- The leak disappears when shaping is done without extra features.
  (This might suggest that the bug is related to the new shape-plan stuff.)
- The leak probably doesn't depend on the font.
  Anyway, here's the font that's used in the code:
http://www.fontsquirrel.com/fonts/download/ChunkFive
- hb_buffer_create/destroy isn't the culprit. The leak disappears when
hb_ft_font_create/destroy is moved outside the loop.

Can you recommend me a revision of HarfBuzz that's been proven stable
enough for daily use?

-Moritz