[base] Use unsigned zero-based refcounts.

This reverts 67cf7a88 to unsigned refcounts, with conditional decrements.

* include/freetype/internal/ftobjs.c (FT_Face_InternalRec,
FT_LibraryRec): Make `refcount' unsigned.
* src/base/ftobjs.c (FT_New_Library, ft_open_face_internal): Keep it zero.
(FT_Done_Library, FT_Done_Face): ... and decrement it conditionally.