Seems to be about what Uniscribe does.  Not exactly.  But close enough.
More consonants will start a new cluster.

A few scripts went way down in failures.  In particular:

  - Devanagari failures went down from 490 to 56.
  - Telugu went down from 113 to 49.

Other scripts went down slightly or didn't change.  New numbers:

BENGALI: 353908 out of 354285 tests passed. 377 failed (0.106412%)
DEVANAGARI: 693572 out of 693628 tests passed. 56 failed (0.00807349%)
GUJARATI: 366485 out of 366506 tests passed. 21 failed (0.00572978%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 950730 out of 951913 tests passed. 1183 failed (0.124276%)
KHMER: 298613 out of 299124 tests passed. 511 failed (0.170832%)
MALAYALAM: 1046881 out of 1048416 tests passed. 1535 failed (0.146411%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271333 out of 271847 tests passed. 514 failed (0.189077%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970524 out of 970573 tests passed. 49 failed (0.00504856%)

Some of the remaining Telugu and Devanagari issues seem to be Uniscribe
eating Anusvara when placed before a non-joiner.  Ouch!

Oops, thinko.

Uniscribe reorders U+0E3A to be after U+0E38 and U+0E39.  We do that by
modifying the ccc for U+0E3A.

Fixes the two remaining Thai failures (see previous commit).

Adjust the list of marks before SARA AM that get the reordering
treatment.  Also adjust cluster formation to match Uniscribe.

With Wikipedia test data, now I see:

  - For Thai, with the Angsana New font from Win7, I see 54 failures out
    of over 4M tests  (0.00129107%).  Of the 54, two are legitimate
    reordering issues (fix coming soon), and the other 52 are simply
    Uniscribe using a zero-width space char instead of an unknown
    character for missing glyphs.  No idea why.  The missing-glyph
    sequences include one that is a Thai character followed by an Arabic
    Sokun.  Someone confused it with Nikhahit I assume!

  - For Lao, with the Dokchampa font from Win7, 33 tests fail out of
    54k (0.0615167%).  All seem to be insignificant mark positioning
    with two marks on a base.  Have to investigate.

Test case was: <U+0D15,U+0D4D,U+0D15,U+0D4A>.

This should address all possible cluster misformations that I had in
mind.

This should fix any instabilities in cluster formation that we were
speculating may happen with surrounding syllables.  Or most of it
perhaps.

Fixes crashes reported with left matra under
non-uniscribe-bug-compatibilty mode.

From:
http://silpa.org.in/pub/tests/hb/ml/ml-harfbuzz-testdata.txt

Generated by:
http://git.savannah.gnu.org/cgit/sinhala.git/plain/utils/gen-unicode-sinhala.py

Merge tests from:
http://git.savannah.gnu.org/cgit/sinhala.git/plain/patches/icu-sinhala-rendering.txt

Improves Bengali and Gurmukhi.  Malayalam regressed a bit.  We will deal
with that later.

This is a bunch of hacks for now.

Improves Bengali a bit.

Gurmukhi failures half now.  Others changed slightly.

Malayalam failures go way down.  Other scripts benefitted slightly too.
Sinhala had one or two test regressions, but...

Fixes 20 out of 48 failing Oriya tests.  Failure rate down to 0.066% now.

Oriya failures down from 0.65% to 0.20%.

Fixes most Malayalam failures.  Down from 1.6% to 0.38% now.  Fixes a
few more in other scripts too.

If x is not constant, we cannot ASSERT_STATIC on it.

Apparently this was approved in Feb 2012.  No font yet.

Althought IndicMatraCategory.txt classifies it as Top_And_Right matra,
it does not have Unicode decomposition, and Uniscribe does not do
anything special about it either.

Gujarati failures down from 0.672% to 0.0130966%.

Now that we break syllables on Halant,ZWNJ, this code can be simplified.

That's really what Uniscribe does, and explains a lot of pecularities of
Halant,ZWNJ before the base.

Sent Telugu from 1% failures to 0.03%.  Improved Kannada and Malayalam
slightly.  Fixed half of Bengali, and did NOT break anything!

Specifically, don't apply 'init' if previous char is a joiner.

Fixes some more of Bengali.

Fixes more of Telugu, Kannada, and Oriya.

May break things (outside Indic...), but we cannot think of any font relying
on this immediately.

Not tuned, just copied from step 2.  Fixes another 0.5% of Kannada
failures.  1% to go.

Fixes a few Devanagari, half of remaining Kannada failures, quarter for
Telugu, and others slightly improved or unchanged.

Fixes 5 Devanagari failures, and no regressions.

Brings down failures with Lohit-Telugu from 57% to 1.40%.

Kannada failures down from 3.5% to 2.93%.

It's not in IndicSyllabicCategory.txt.  Fixes most of Gurmukhi failures.
Failures down from 7.7% to 0.222%!

For Khmer, all consonants are subjoining.  No need to look in the font.
We were looking in the wrong order anyway.