Fixes assert fail in https://github.com/behdad/harfbuzz/issues/161
with libharfbuzz-fuzzing.

Unused currently.  To be used for Syriac stretch implementation.

https://github.com/behdad/harfbuzz/issues/141

https://bugzilla.mozilla.org/show_bug.cgi?id=1215894

From https://bugzilla.mozilla.org/show_bug.cgi?id=1215894

Borrowed from https://bugzilla.mozilla.org/show_bug.cgi?id=1215411

Needed for upcoming merge-clusters fix.

And add check to FLAG()

See comment.

Micro-tests added.

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.

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.

If pre-base reordering Ra is NOT formed (or formed and then
broken up), we should consider that Ra as base.  This is
observable when there's a left matra or dotreph that positions
before base.

Now, it might be that we shouldn't do this if the Ra happend
to form a below form.  We can't quite deduce that right now...

Micro test added.  Also at:

https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=186#c29

Sometimes font designers form half/pref/etc consonant forms
unconditionally and then undo that conditionally.  Try to
recover the OT_H classification in those cases.

No test number changes expected.

No test numbers change.

Normally if you want to, say, conditionally prevent a 'pref', you
would use blocking contextual matching.  Some designers instead
form the 'pref' form, then undo it in context.  To detect that
we now also remember glyphs that went through MultipleSubst.

In the only place that this is used, Uniscribe seems to only care
about the "last" transformation between Ligature and Multiple
substitions.  Ie. if you ligate, expand, and ligate again, it
moves the pref, but if you ligate and expand it doesn't.  That's
why we clear the MULTIPLIED bit when setting LIGATED.

Micro-test added.  Test: U+0D2F,0D4D,0D30 with font from:

[1]
https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=186#c29

Apparently they can intermix with other OT_A.

Test: U+0915,A8E2,1CD0

Tests: U+0967,0951 U+0031,093F

No functional change intended.

Unicode 6.x IndicSyllableCategory categorizes them as
placeholders, but they can subjoin.

In anticipation of adding more characters to that class of clusters.

It's spacing, but otherwise the same as the other ones.

Up to two of them come after all OT_A characters.

Also adjust U+0953,0954 handling.