[cairo] FreeType refcounting

[Behdad Esfahbod]

The new FreeType 2.4.2 adds the following functions:

    - Two new functions,  `FT_Reference_Library' (in FT_MODULE_H)  and
      `FT_Reference_Face'  (in  FT_FREETYPE_H),  have  been  added  to
      simplify life-cycle management.  A counter gets initialized to 1
      at the  time an  FT_Library (or  FT_Face) structure  is created.
      The  two  new   functions  increment  the  respective   counter.
      `FT_Done_Library' and `FT_Done_Face' then only destroy a library
      or face if the counter is 1, otherwise they simply decrement the
      counter.

Would be nice to use them when available.