So it's easier to cross-reference from output of print-indic-table.

Fixes https://github.com/behdad/harfbuzz/issues/552

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

Fixes https://github.com/behdad/harfbuzz/issues/528
"Kannada JIHVAMULIYA and UPADHMANIYA insert dotted circles"

Test stats remain unchanged, except for Malayalam, which we investigate:

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: 1047584 out of 1048334 tests passed. 750 failed (0.0715421%)
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%)

Myanmar, compared to Windows 10 mmrtext.ttf:

MYANMAR: 1123865 out of 1123883 tests passed. 18 failed (0.00160159%)

Touch code just enough to preserve previous syllable structure
and functionality as closely as possible.  Many further cleanups
coming later.

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

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.

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

Handles Tai Tham, Cham, and New Tai Lue for now.

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

To be used in upcoming commits.

No panic, we reeally insert dotted circle when it's absolutely broken.

Fixes most of the dotted-circle cases against Uniscribe. (for Devanagari
fixes 80% of them, for Khmer 70%; the rest look like Uniscribe being
really bogus...)

I had to make a decision.  Apparently Uniscribe adds one dotted circle
to each broken character.  I tried that, but that goes wrong easily with
split matras.  So I made it add only one dotted circle to an entire
broken syllable tail.  As in: "if there was a dotted circle here, this
would have formed a correct cluster."  That works better for split
stuff, and I like it more.

To room for more allocations, coming.

This concludes the Indic shape_plan work.  May do for Arabic also...

Move all the logic that needs to eventually move into the indic table
into hb-ot-shape-complex-indic-private.hh.

Apparently this was approved in Feb 2012.  No font yet.

Fixes most failures of Oriya, and improves others a bit.

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.