[HarfBuzz] How to get hb_face_t and hb_font_t without Freetype?

[Eli Zaretskii]

And one more question.  The shaping example in the HarfBuzz docs does
this after shaping:

     x_offset = glyph_pos[i].x_offset / 64.0;
     y_offset = glyph_pos[i].y_offset / 64.0;
     x_advance = glyph_pos[i].x_advance / 64.0;
     y_advance = glyph_pos[i].y_advance / 64.0;

Where do those 64.0 factors come from?  IOW, I guess the question is
in what units are the members of hb_glyph_position_t measured, and how
to scale that to the pixels of the display?

TIA