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!

This reorders glyphs within the cluster to a nominal order.  This should
have no visible effect on the output, but helps with testing, for
getting the same hb-shape output for visually-equal glyphs for each
cluster.

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

Except that there may be a ZWNJ before a Register Shifter.

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).

Fixes issue reported by Khaled Hosny with his Hussaini Nastaleeq font
and sequences like those added in the previous commit.

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 Khmer failures with Daun Penh font from 36% to 20%.

Khmer failures down from 58% to 36%.

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.

This is a hack for now, to get us going with Khmer.  This will be
refactored properly later to move the complex logic into complex
shapers.

We are going to split matras without a Unicode decompositions in a way
that the second half takes the codepoint of the whole matra.  So,
position them where the second half is supposed to end up.

Apparently Uniscribe allows these, and they are used in some Khmer fonts
shipped with Windows, namely, Daun Penh.