Scans a FreeType face and returns the set of encoded Unicode chars.
This scans
several encoding tables to build as complete a list as possible.
If 'blanks' is not 0, the glyphs in the font are examined and any blank glyphs
not in 'blanks' are not placed in the returned FcCharSet.
spacing receives the computed spacing type of the
font, one of FC_MONO for a font where all glyphs have the same width,
FC_DUAL, where the font has glyphs in precisely two widths, one twice as
wide as the other, or FC_PROPORTIONAL where the font has glyphs of many
widths.