hb_font_set_scale() now sets the value to be used to represent a unit
pixel. For example, if rendering a 10px font with a 26.6 representation,
you would set scale to (10 << 6). For 10px in 16.16 you would set it to
(10 << 16). This space should be the same space that the get_glyph_metrics
and get_kerning callbacks work in.