Actual code follows in a later commit.
* include/freetype/ftautoh.h: Document `no-stem-darkening' and
`darkening-parameters'.
* src/autofit/afmodule.h: New fields `no_stem_darkening' and
`darken_params'.
* src/autofit/afmodule.c (af_property_set, af_property_get):
Handle them.
(af_autofitter_init): Initialize them.