… description.

- The returned value as width/height/etc. of the glyph extents (or bounding
  box), not "of the font" (which doesn't mean much).
- Adding some definition for ascent and descent. This text is straight out
  copied from Pango documentation comments in pango/pango-types.h.
- I don't see why we were negating the descent value. Let's keep the value sign
  as defined in Pango.