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.
Exposes the non-atomicity of user_data opertaions at this time because
we call finish() while still locked and modifying the object. In fact,
I'm surprised that it doesn't deadlock. It should.