New numbers (down from 39):

KHMER: 299086 out of 299124 tests passed. 38 failed (0.0127038%)

New numbers (down from 44):

KHMER: 299085 out of 299124 tests passed. 39 failed (0.0130381%)

This is how 1471cfee should have
been done.

Used by Sinhala only.

Uniscribe seems to allow that.  New numbers:

KHMER: 299080 out of 299124 tests passed. 44 failed (0.0147096%)

Towards fixing https://github.com/harfbuzz/harfbuzz/issues/667
The Khmer spec is different enough from other Indic ones to require
its own grammar.

No change in functionality.  Test numbers are:

BENGALI: 353725 out of 354188 tests passed. 463 failed (0.130722%)
DEVANAGARI: 707307 out of 707394 tests passed. 87 failed (0.0122987%)
GUJARATI: 366355 out of 366457 tests passed. 102 failed (0.0278341%)
GURMUKHI: 60729 out of 60747 tests passed. 18 failed (0.0296311%)
KANNADA: 951300 out of 951913 tests passed. 613 failed (0.0643966%)
KHMER: 299071 out of 299124 tests passed. 53 failed (0.0177184%)
MALAYALAM: 1048136 out of 1048334 tests passed. 198 failed (0.0188871%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271662 out of 271847 tests passed. 185 failed (0.068053%)
TAMIL: 1091754 out of 1091754 tests passed. 0 failed (0%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)

Fixes https://github.com/behdad/harfbuzz/issues/528

In Oriya, a ZWJ/ZWNJ might be added before candrabindu to encourage
or stop ligation of the candrabindu.  This is clearly specified in
the Unicode section on Oriya.  Allow it there.  Note that Uniscribe
doesn't allow this.

Micro tests added using Noto Sans Oriya draft.

No changes in numbers.  Currently at:

BENGALI: 353725 out of 354188 tests passed. 463 failed (0.130722%)
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: 951190 out of 951913 tests passed. 723 failed (0.0759523%)
KHMER: 299070 out of 299124 tests passed. 54 failed (0.0180527%)
MALAYALAM: 1048147 out of 1048334 tests passed. 187 failed (0.0178378%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271662 out of 271847 tests passed. 185 failed (0.068053%)
TAMIL: 1091753 out of 1091754 tests passed. 1 failed (9.15957e-05%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)

Only allow it on DOTTED CIRCLE.  No effect on test numbers.

Test: U+0930,094D,00A0

No functional change.

In fact, the previous grammar was ambigious.  No functional
change.

In anticipation of adding more characters to that class of clusters.

Up to two of them come after all OT_A characters.

According to Roozbeh, there are valid combinations in Unicode
proposals for up to three.  Previously we were allowing up to two.

Followup from 6e613f33.

Seems to be working just fine!

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.

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.

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

Mostly affects Myanmar, but also Tai Tham, Javanese, and Cham.  The
latter three are untested (no fonts!).

With FreeSerif, it seems that the 'ccmp' feature does ligature
substituttions.  That was then causing syllable match failures.  We now
find syllables before any features have been applied.

Test sequence: U+0D9A,U+0DCA,U+200D,U+0DBB,U+0DCF