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.

Fixes 1.5% more failures for Telugu, 2% for Kannada.
Breaks one test in Devanagari.

Fixes another 5% of Kannada failures.

Fixes most failures of Oriya, and improves others a bit.

Following what the spec says.

Brings down Telugu failures from 40% to 3.75%, and Kannada failures from
44% to 10%.  Does NOT affect other scripts' test results.

POS_BASE can disappear if base ligated backward.  Define base as last
with position not after base.

Fixes a few hundred of Sinhala failures with Iskoola Pota.

In Sinhala, Reph is formed only explicitly, by the presence of a ZWJ.

Really this time.

Fixes another 18 Khmer tests.

For example: U+915,U+200c,U+93f

Fixes last Tamil failure!

Tamil has only about 150 failures now!

It's a visual Repha.

Still not positioning logical Repha as occurs in Malayalam.

Another 200 Khmer failures fixed.  547 to go.  That's better than
Devanagari!

Down another 500 or so Khmer failures!

In such scripts (ie. Khmer), a ZWJ/ZWNJ shouldn't stop the search for
base.  So, instead just choose the first consonant as base directly.

Test sequence:
U+1798,200c,U+17C9,U+17D2,U+179B,U+17C1,U+17C7

Mark stuff after a pre-base reordering Ro 'cfar'.  Used in Khmer.
This allows distinguishing the following cases with MS Khmer fonts:

  U+1784,U+17D2,U+179A,U+17D2,U+1782
  U+1784,U+17D2,U+1782,U+17D2,U+179A

In Khmer, a final subjoined consonant or independent vowel can occur
after matras.  This final subjoined thing should NOT be reordered to
before the matra even though it's subjoined.

Fixes another 1k of the Khmer failures.  Not much left really.

Amend the syllable structure to allow a final subscripted consonant
(Coeng+C) and a final subscripted independent vowel (Coeng+V).
Fixes another 2k of Khmer failures.

No characters recategorized yet.  No semantic change.

Normally, we attach the Halant to the previous character and move it
with it.  For after-base consonants however, the Halant "belongs" to the
consonant after, so attach it so.

This fixes Bengali sequences involving post-base consonant Ya, which
should ligate with the Halant to form Ya Phala, but previously a
reordered matras was blocking the ligation.

Seems like this is what Uniscribe is doing, and does not break any fonts
we tested (with Devanagari, Malayalam, Khmer, and Bengali), while fixing
some Ra Phala sequences for Bengali with Vrinda.  Fixes another 2% of
Bengali failures (a couple more to go).

Uniscribe does not apply 'kern' in the Indic module.  Some of the Khmer
fonts they ship have small adjustments in the 'kern' table.  Disable
'kern' in the Indic module under Uniscribe bug compatibility mode.

Fixes some 10% of the Khmer failures.  Remains under 3% (excluding
dotted-circle ones).

In a new callback...  Currently unused by all complex shapers.

Mark them the same as the Register Shifters for now.  Need to rename
that category to something more sensible after all is settled.

Fixes another percent of Khmer failures.  Down to under 3%!

We were doing that only immediately after base.

Fixes another percent in the Khmer failures.  About three more to go...

Fixes another 6% of the Khmer failures.

Brings down Malayalam failures from 14% down to 3%.

No semantic change.

Almost...

No reordering yet.

Should not be any semantic change.  In preparation for implementing
pre-base reordering Ra.