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.

Untested.

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.

This is a hack for now.  Will be fixed when we do complex-shaper-driven
normalization properly.

The results with or without decomposition are the same, but Uniscribe
does not normalize, so this matches better.

In Sinhala, Rakar is formed by Al-Lakuna,ZWJ,Ra.  If you put that at the
end of a Consonant,Matra syllable, you get a dotted-circle from
Uniscribe.  Apparently adding a ZWJ before the Al-Lakuna "fixes" that.
And people have been encoding that sequence...  So, allow a forced
"ZWJ,Virama,ZWJ,Ra" sequence at the of syllables.

Fixes some 100 or more of Sinhala failures.  Now at 622 only (0.23%).

The sequence <ZWJ,Al-Lakuna,ZWJ> is used in Sinhala to explicitly ask
for Rakar.  Fixes two-thousand Sinhala tests.  Not many left.

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.

Makes no visual difference.

Fixes most of the failures.  Down from 15% to 1.3%!

Fixes all Tibetan failures.  All 180k of them!

Merges back Hangul into the default shaper.

Really this time.

Fixes another 18 Khmer tests.

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

Fixes last Tamil failure!

Just in case, for the times we do out-of-bounds access.

jk

Fixes 1 more Devanagari test!

Fixes 2 out of 560 Devanagari failures.  AND:
Fixes 1 out of 2 Tamil failures.

Refactoring Indic machin.  No semantic change.

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!

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