debug mode of template instantiation

        * src/cff/cffparse.c: fixed the CFF table loader. It didn't accept
        empty arrays, and this prevented the loading of certain fonts.

        * src/pshinter/pshalgo2.c, src/pshinter/pshalgo3.c: adding fix to
        prevent seg fault when hints are provided in an empty glyph !!

        * include/freetype/ftimage.h: removed incorrect "zft_" definitions
        and updated constants documentation comments

        * include/freetype/freetype.h (FT_FaceRec): updating documentation
        comment. The "descender" value is always *negative*, not positive !