It's both faster and produces smaller code.  Now I feel stupid for
not writing it this way before.

Only if the font doesn't support it.  Ie, this gives the user to
use non-Unicode codepoints as private values and return a meaningful
glyph for them.  But if it's invalid and font callback doesn't
like it, and if font has U+FFFD, show that instead.

Font functions that do not want this automatic replacement to
happen should return true from get_glyph() if unicode > 0x10FFFF.

Replaces https://github.com/behdad/harfbuzz/pull/27

Test passes now.

Previous code was broken logically, but harmless.

Fixes https://github.com/behdad/harfbuzz/pull/45

From http://github.com/behdad/ucdn

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.