[Indic] Minor refactoring
[GSUB/GPOS] Const correctness
[Indic] exclude ligatures when matching on Indic category

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%).
[Indic] Improve check on ligatures

Only skip actual ligatures, not marks in-between ligature components.
Minor refactoring
When removing zero-width marks, don't remove ligatures

If a mark ligated, it probably should NOT be removed.
[Indic] Change "unknown" position to end of syllable
[Indic] Improve Sinhala base algorithm and reph positioning

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%).
[Indic] End Vowel-based syllable at ZWJ

One Devanagari test regressed, plus 10 Malayalam (at 1545 now).

Fixed 120 Sinhala failures.  Now at 208 (0.0765136%).
[Indic] Further adjust base algorithm for Sinhala

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.
[Indic] In Sinhala, form forced Reph even if no other consonant found

Fixes another 10 Sinhala failures.  Down to 148 (0.0544424%).
[Indic] Don't reposition Khmer Bindu

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%).
[Indic] Position Khmer U+17CE

Fixes another 6 Khmer failures.  Now at 136 (0.0454661%).
[Indic] Fix minor bug in pre-base Ra positioning
[Indic] Reposition Khmer prebase-reordering Ra around split matras

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%).
[Indic] Recategorize Khmer various signs as top matras

Khmer failures down from 39 to 31 (0.0103636%).
[Indic] Unmark U+17D1 KHMER SIGN VIRIAM to NOT be a Virama

Fixes another 1 Khmer failure.  Down to 30 (0.0100293%) now.
[Indic] Limit matras to 4 per syllable

Also limit joiners.

This limits our syllable length to a constant, and is
closer to what Uniscribe does anyway.

Two Devanagari tests regressed, but who cares about tests with 20
joiners in a row?!  Devanagari at 57 (0.00821766%) now.
[Indic] Implement Reph+Ya-Phalaa interaction

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%)
[Indic] Better position left-matra in Malayalam

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%)