Useful for benchmarking, to avoid buffer serialization overhead (which
seems to by far dominate shaping!)

Supports ft and ot right now.  hb-view currently not rendering with ot.
Will fix after some clean up.

He's been my shadow for all Indic-related changes in the last
few months.

These were introduced in a498565c,
but IndicSyllabicCategory has had the correct value already, so the
special code was never needed.

Touch code just enough to preserve previous syllable structure
and functionality as closely as possible.  Many further cleanups
coming later.

There may be more.  There are members that are by definition
redundant or reserved and not needed, NOT what we *currently*
don't use.

I'm sure there's more...

Add hb_ot_layout_language_get_required_feature_index() again, which
is used in Pango.  This was removed in
da132937 in favor of
hb_ot_layout_language_get_required_feature().

API changes:

  - Added hb_ot_layout_language_get_required_feature_index back.

Was from 6.2.0.  It's a no-op.  Committing for the record.

Old table was from 6.2.  Remove hard-coded Mongolian and Phags-pa data.
This completes support for new scripts Manichian and Psaltar Pahlavi.

In anticipation of Unicode 7.0 data coming in the next commit.

No functional change.

No output change.