Hookup some Indic data to it.  More to come.

We need the font for glyph lookup during GSUB pauses in Indic shaper.
Could perhaps be avoided, but at this point, we don't mean to support
separate substitute()/position() entry points (anymore), so there is
no point in not providing the font to GSUB.

To be moved to initial_reordering next...

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

Comes with some 10% speedup for Devanagari even!

If there is no GPOS, zero mark advances.

If there *is* GPOS and the shaper requests so, zero mark advances for
attached marks.

Fixes regression with Tibetan, where the font has GPOS, and marks a
glyph as mark where it shouldn't get zero advance.

Enabled for all shapers except for Indic.

Add a shaper class struct.

When we removed the separate Hangul shaper, the specific normalization
preference of Hangul was lost.  Fix that.  Also, the Thai shaper was
copied from Hangul, so had the fully-composed normalization behavior,
which was unnecessary.  So, fix that too.

The font is forming a post-base consonant in some samples, and Uniscribe
positions top matra on the post-base.  Do the same.

Gurmukhi failures down from 59 to 41 (0.0674242%).

Just put it before base, which is what's expected.

Malayalam failures down from 1559 to 1197 (0.114172%).

BENGALI: 353988 out of 354285 tests passed. 297 failed (0.0838308%)
DEVANAGARI: 693571 out of 693628 tests passed. 57 failed (0.00821766%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 950956 out of 951913 tests passed. 957 failed (0.100534%)
KHMER: 299094 out of 299124 tests passed. 30 failed (0.0100293%)
MALAYALAM: 1047219 out of 1048416 tests passed. 1197 failed (0.114172%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271699 out of 271847 tests passed. 148 failed (0.0544424%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970524 out of 970573 tests passed. 49 failed (0.00504856%)

The sequence Ra,H,Ya in Bengali is ambigious and Unicode encoded that to
get Ya-Phalaa, one would place ZWJ before Halant.  Ie. a ZWJ,H sequence
requests subjoining, while a H,ZWJ requests Half form.  Implement that.

Bengali failures go down from 377 to 297 (0.0838308%).
Gujarati is down by 4 to 17 (0.0046384%).
Kannada is down by 226 to 957 (0.100534%).

Current status:

BENGALI: 353988 out of 354285 tests passed. 297 failed (0.0838308%)
DEVANAGARI: 693571 out of 693628 tests passed. 57 failed (0.00821766%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 950956 out of 951913 tests passed. 957 failed (0.100534%)
KHMER: 299094 out of 299124 tests passed. 30 failed (0.0100293%)
MALAYALAM: 1046857 out of 1048416 tests passed. 1559 failed (0.148701%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271699 out of 271847 tests passed. 148 failed (0.0544424%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970524 out of 970573 tests passed. 49 failed (0.00504856%)

Fixes another 1 Khmer failure.  Down to 30 (0.0100293%) now.

Khmer failures down from 39 to 31 (0.0103636%).

In Khmer coeng model, a V,Ra can go *after* matras.  If it goes after a
split matra, it should be reordered to *before* the left part of such matra.

Khmer failures down from 136 to 39 (0.0130381%).

Fixes another 6 Khmer failures.  Now at 136 (0.0454661%).

Khmer Bindu doesn't like to move to syllable end.  Leave it where it
was.

Brings down Khmer failures from 510 to 142 (0.047572%).

Fixes another 10 Sinhala failures.  Down to 148 (0.0544424%).

Apparently if there is C,V,ZWJ,C, the first C will be base, but if
it's C,ZWJ,V,C, the second one will be.

Note that Uniscribe implements this differently, by breaking syllable in
the case of C,ZWJ,V,C and putting the first consonant in one syllable
and the rest in the next syllable.

Sinhala failures down from 208 to 158 (0.0581209%).  No changes to
Khmer.

Sinhala does not have half forms.  And most (all?) consonants can be
base, except when preceded by ZWJ, which would request a subjoined form.
Hence switch the base algorithm to categorize with Khmer, start search
at start, and stop at a ZWJ.

Also, mark all pos=base consonants after base to be subjoined.  Mark
base itself to have pos=base.

Finally, adjust Sinhala's reph position to after-main.

Brings down Sinhala failures from 455 to 328 (0.120656%).

Only skip actual ligatures, not marks in-between ligature components.

If, say, a H,ZWJ,C ligature was formed, we don't want the code to detec
that as a Halant.  So, ignore ligatures when matching category in
final_reordering.

Sinhala failures down from 514 to 455 (0.167374%).

We now limit syllable lengths in the machine.  No need to match here.

Test case was: <U+0D15,U+0D4D,U+0D15,U+0D4A>.

This should address all possible cluster misformations that I had in
mind.

This should fix any instabilities in cluster formation that we were
speculating may happen with surrounding syllables.  Or most of it
perhaps.

Fixes crashes reported with left matra under
non-uniscribe-bug-compatibilty mode.