The GSUBGPOS part still missing.

Filling in.

Should have no visible effect.

To be used in the Indic shaper later.  Unused for now.

As reported by datao zhang on the mailing list.

https://www.microsoft.com/typography/otfntdev/devanot/shaping.aspx

Also fix cairo cluster generation.

Better catches cluster errors.

Apparently that's what Kazuraki uses to form vertical ligatures,
which suggests that it's what Adobe does.

As was reported to me, glib-mkenum does not understand named enums,
so remove for now.

This changes all the HB_DIRECTION_* enum member values, but is
nicer, in preparation for making hb_segment_properties_t public.

For some reason it doesn't quite work with IranianNastaliq, but
that looks like a font issue.

Also remove shaper_options argument to hb_shape_full().  That was
unused and for "future".  Let it go.

More shaper API coming in preparation for plan/planned API.