[HarfBuzz] Inheriting/copying font functions

[Tom Hacohen]

Dear Behadad,

After your recent changes (which removed the
hb_font_funcs_get_contour_point_func and the like) and since there's no
way to inherit/copy other func functions. It's now impossible to
implement only part of the font functions and use Harfbuzz's default for
freetype for the rest. Correct me if I'm wrong, but that's at least from
my observation on the exposed API.

Furthermore, do you really think that letting the users set user data
with keys is the way to go? I think just solving the problem of internal
user_data and external user data by having two of those is more than
enough and probably a lot faster. I doubt that anyone will ever need
more than 1 user data once you do the internal/external separation, so
we'll probably just lose precious cpu cycles here.

Thanks,
Tom.