Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly. This makes it easier to
refactor headers later on without breaking compatibility.
Remove hb_ft_get_font_funcs() as it cannot be used by the user anyway.
Add hb_ft_font_set_funcs(). Which will make the font internally use
FreeType. That is, no need for the font to have created using the
hb-ft API. Just create using hb_face_create()/hb_font_create() and
then call this on the font (after having set font scale). This
internally creates an FT_Face and attached to the font.
Behdad Esfahbod