Extract underline positions from PostScript metrics tables

Extract underline positions from PostScript metrics tables

* src/sfntfont.c (struct sfnt_font_desc): Introduce fields where
the font's underline position is recorded.
(sfnt_enum_font_1): Compute the underline position with
information in the post table whenever it exists.
(sfntfont_open): Scale the recorded position and save it into
the font object.