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%)

Found using `codespell -i 3 -w -I '../harf-whitelist.txt'`
whereas the whitelist contents were:
```
beng
iff
pres
ot
te
teh
```

In ten years we never used them...

No flag is safe, over time... See previous commit.

Fixes https://github.com/behdad/harfbuzz/issues/435

* Clarified wording of syllable/cluster behavior in Uniscribe in final_reordering; changed one other probable typo.

* Additional syllable/cluster swap in comments for final reordering and for initial-reordering matra decomposition.

Fixes https://github.com/behdad/harfbuzz/issues/509

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

Mark it as matra below to allow the sequence U+0A15, U+0A51, U+0A47.
Oh well...

Fixes https://github.com/behdad/harfbuzz/issues/524

Fixes https://github.com/behdad/harfbuzz/issues/538

Fixes https://github.com/behdad/harfbuzz/issues/294

Also fixes a bunch of other Indic issues.  Test results after:

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: 60732 out of 60747 tests passed. 15 failed (0.0246926%)
KANNADA: 951201 out of 951913 tests passed. 712 failed (0.0747968%)
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%)

Before:

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: 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: 1091753 out of 1091754 tests passed. 1 failed (9.15957e-05%)
TELUGU: 970555 out of 970573 tests passed. 18 failed (0.00185457%)

This has no security implications whatsoever since we always keep
and extra element at the end of buffer, just in case.

Discovered by oss-fuzz
CC https://github.com/behdad/harfbuzz/issues/139
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=660

The decomposition is very obscure and unlikely to help
any fonts.  Just remove it since Uniscribe probably doesn't
do this either.

Fixes https://github.com/behdad/harfbuzz/issues/382

Part of https://github.com/behdad/harfbuzz/issues/382

This is now added to USE:
https://github.com/behdad/harfbuzz/pull/383

See:
https://github.com/behdad/harfbuzz/issues/381
https://github.com/behdad/harfbuzz/issues/382

This reverts commit 805f76fc.

Need to investigate why having this rule (with the reversed order)
fixes decomposition as reported by punchcutter:
https://github.com/behdad/harfbuzz/issues/381#issuecomment-269237796

This was not used BTW since Chakma doesn't go through Indic shaper
anymore.

See:
https://github.com/behdad/harfbuzz/issues/381
https://github.com/behdad/harfbuzz/issues/382

New approach to fix this:
https://github.com/behdad/harfbuzz/commit/69f9fbc4200442a35484d3c790ae8f4979be5d60

Previous approach was reverted as it was too broad.  See context:
https://github.com/behdad/harfbuzz/issues/347#issuecomment-267838368

With U+05E9,U+05B8,U+05C1,U+05DC and Arial Unicode, we now (correctly) disable
GDEF and GPOS, so we get results very close to Uniscribe, but slightly different
since our fallback position logic is not exactly the same:

Before:		[gid1166=3+991|gid1142=0+737|gid5798=0+1434]
After:		[gid1166=3+991|gid1142=0@402,-26+0|gid5798=0+1434]
Uniscribe:	[gid1166=3+991|gid1142=0@348,0+0|gid5798=0+1434]

Bending to clang warnings...
https://bugs.chromium.org/p/chromium/issues/detail?id=593057

New API:
- hb_font_get_nominal_glyph_func_t
- hb_font_get_variation_glyph_func_t
- hb_font_funcs_set_nominal_glyph_func()
- hb_font_funcs_set_variation_glyph_func()
- hb_font_get_nominal_glyph()
- hb_font_get_variation_glyph()

Deprecated API:
- hb_font_get_glyph_func_t
- hb_font_funcs_set_glyph_func()

Clients that implement their own font-funcs are encouraged to replace
their get_glyph() implementation with a get_nominal_glyph() and
get_variation_glyph() pair.  The variation version can assume that
variation_selector argument is not zero.

Hopefully
fixes https://github.com/behdad/harfbuzz/issues/214