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.

As reported by Jonathan Kew on the list.

Testing with old Deva spec this reduces failures.
Test sequence: U+0915,U+094D,U+0930.

This reverts commit 0831061e.

Bengali is at *just* above 5% now.

Error down from 20% to 7%.

Yep, on Bengali now!

For dotted-circle independent clusters, Uniscribe does no Reph shaping
for the exact sequence Ra+Halant+25CC.  Which also is the only possible
sequence with 25CC at the end.