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.

No functional change.

No functional change.

No functional change.

We don't need Indic table for those.

-#define indic_offset_total 4416
+#define indic_offset_total 3816

-}; /* Table occupancy: 60% */
+}; /* Table occupancy: 69% */

No functional change.

HB_VERSION_CHECK's comparison was originally written wrongly
by mistake.  When API tests were written, they were also written
wrongly to pass given the wrong implementation... Sigh.

Given the purpose of this API, there's no point in fixing it
without renaming it.  As such, rename.

API changes:

  HB_VERSION_CHECK -> HB_VERSION_ATLEAST
  hb_version_check -> hb_version_atleast

https://bugzilla.mozilla.org/show_bug.cgi?id=986802
Fixes https://github.com/behdad/harfbuzz/pull/39

API Change:

-hb_ot_layout_language_get_required_feature_index
+hb_ot_layout_language_get_required_feature

New API takes an extra pointer argument.  Pass NULL in to get
behavior of previous API.

Reworked by behdad

Such that the complexity of the algorithm used is clear at
call site.

In an effort to make the algorithm used clear.

Still needs update to joining table to fully work.