[autofit] Don't depend on 'hb-ft'.

The circular dependency is still there, but at least we no longer depend on
the HarfBuzz API that is only present if HarfBuzz has been built with
FreeType support, making the bootstrapping a bit easier.

* src/autofit/ft-hb.c, src/autofit/ft-hb.h: New files, providing
`_hb_ft_font_create`, which is more or less a verbatim copy of the
corresponding HarfBuzz code from file `hb-ft.cc`.

* src/autofit/afglobal.c (af_face_globals_new): Use it.
* src/autofit/afshaper.h: Don't include `hb-ft.h` but `ft-hb.h`.
* src/autofit/autofit.c: Include `ft-hb.c`.

* LICENSE.TXT: Updated.