coretext: Implement pango_font_get_hb_font
This will let us expose font functionality without plumbing everyhing through the pango api.
harfbuzz has apis that should help:
HB_EXTERN hb_face_t *
hb_coretext_face_create (CGFontRef cg_font);
HB_EXTERN hb_font_t *
hb_coretext_font_create (CTFontRef ct_font);
HB_EXTERN CGFontRef
hb_coretext_face_get_cg_font (hb_face_t *face);
HB_EXTERN CTFontRef
hb_coretext_font_get_ct_font (hb_font_t *font);