Both Indic and SEA seem to do it just fine, but SEA is much
simpler.

For scripts without an old/new spec distinction, use zero-context=false.
This changes behavior in Sinhala / Khmer, but doesn't seem to regress.
This will be useful and used in Javanese.

Previously we planted this into the mode used for Khmer.  There's not
really much in common between the two, so separate again.

Similar to 190c8f2b but for
Sinhala.

It's cheap.

Shouldn't change behavior at all, but is faster / more robust.

Whic means these twp are applied per-syllable now.  Apparently
in some Khmer fonts the clig interacts with presentation features.

Test case: U+1781,U+17D2,U+1789,U+17BB,U+17C6 with Mondulkiri-R.ttf
should produce one big ligature.

If a glyph can be both blwf and pref, we were wrongly sorting it
in the post position instead of below position.

Test case: U+09AC,U+09CD,U+09A6 with Lohit-Bengali 2.5.3.

Commit 6b65a76b.  "end" was becoming
negative.  Was trigerred by Lohit-Kannada 2.5.3 and the sequence:
U+0CB0,U+200D,U+0CBE,U+0CB7,U+0CCD,U+0C9F,U+0CCD,U+0CB0,U+0C97,U+0CB3
Two glyphs were being duplicated.

First, we were abusing OT_VD instead of OT_A.  Fix that
but moving OT_A in the grammar where it belongs (which
is different from what the spec says).

Also, only allow medial consonants after all other
consonants.  This doesn't affect any current character.

Finally, fix Halant attachment in presence of medial
consonants.  Again, this currently doesn't affect any
sequence.

I lied.  There's Gurmukhi U+0A75 which is Consonant_Medial.
Uniscribe allows one of those in each of these positions:
before matras, after matras and before syllable modifiers,
and after syllable modifiers!  We currently just allow
unlimited numbers of it, before matras.

Unicode 6.2.0 Section 16.2 / Figure 16.3 says:

"For backward compatibility, between Arabic characters a ZWJ acts just
like the sequence <ZWJ, ZWNJ, ZWJ>, preventing a ligature from forming
instead of requesting the use of a ligature that would not normally be
used. As a result, there is no plain text mechanism for requesting the
use of a ligature in Arabic text."

As such, we flip internal zwj to zwnj flags for GSUB matching, which
means it will block ligation in all features, unless the font
explicitly matches U+200D glyph.  This doesn't affect joining behavior.

The merge_clusters there was totally broken.

Apparently Uniscribe Tamil shaper doesn't ship chubby clusters
for Tamil.  Adjust to that.

Bug 70509 - Candrabindu+Visarga doesn't work in Devanagari
https://bugs.freedesktop.org/show_bug.cgi?id=70509

We categorize both bindus and visarga as syllable-modifiers.
OT spec doesn't actually say what characters go in the syllable
modifier category, and allows one.  We just allow up to two now.

Test case: U+0930,U+0941,U+0901,U+0903

Uniscribe currently doesn't support that and produces a
dotted circle.

*Shouldn't* break anything.

Rachana-0.6 implements dot-reph by ligation, so we shouldn't move it.
Uniscribe doesn't either.  Test case:

  U+0D4E,U+0D1A,U+0D4D,U+0D1A,U+0D4D

Seems to better match Uniscribe.

Note: NotoSansTelugu-Regular has kern feature, so this fixes most of the
positioning failures there, except for the kern pairs blocked by a
(non-)joiner, in which case we (correctly) kern, but Uniscribe doesn't.

More like Uniscribe...  We still allow user-defined features to
work across syllables, but not pres,blws,abs,psts,etc.

This "regressed" Sinhala numbers by 11.  These are cases were
there's Consonant followed by Ra,Halant,ZWJ at the of text.
The Ra,Halant,ZWJ ends up forming reph, which is wrong...
But before we were also ligating that reph with the previous
consonant.  That's even more wrong.  That's also what Uniscribe
does.

Current numbers:

BENGALI: 353732 out of 354188 tests passed. 456 failed (0.128745%)
DEVANAGARI: 707307 out of 707394 tests passed. 87 failed (0.0122987%)
GUJARATI: 366349 out of 366457 tests passed. 108 failed (0.0294714%)
GURMUKHI: 60732 out of 60747 tests passed. 15 failed (0.0246926%)
KANNADA: 951030 out of 951913 tests passed. 883 failed (0.0927606%)
KHMER: 299070 out of 299124 tests passed. 54 failed (0.0180527%)
MALAYALAM: 1048140 out of 1048334 tests passed. 194 failed (0.0185056%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271655 out of 271847 tests passed. 192 failed (0.070628%)
TAMIL: 1091753 out of 1091754 tests passed. 1 failed (9.15957e-05%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)

It can come either at the end(ish!) of the syllable, or independently.
When independent, it accepts a few bits and pieces.

...while some don't!

Improved Bengali, Devanagari, Gurmukhi, Malayalam.

Updated numbers:

BENGALI: 353732 out of 354188 tests passed. 456 failed (0.128745%)
DEVANAGARI: 707307 out of 707394 tests passed. 87 failed (0.0122987%)
GUJARATI: 366349 out of 366457 tests passed. 108 failed (0.0294714%)
GURMUKHI: 60732 out of 60747 tests passed. 15 failed (0.0246926%)
KANNADA: 951030 out of 951913 tests passed. 883 failed (0.0927606%)
KHMER: 299070 out of 299124 tests passed. 54 failed (0.0180527%)
MALAYALAM: 1048134 out of 1048334 tests passed. 200 failed (0.0190779%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271666 out of 271847 tests passed. 181 failed (0.0665816%)
TAMIL: 1091753 out of 1091754 tests passed. 1 failed (9.15957e-05%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)