Towards disabling ZWJ-skipping in certain GPOS contexts.

Part of https://github.com/flutter/flutter/issues/16886

Sorry for the noise, downstream custom builders.  Please adjust.

To help TSan and be more "correct".

Oriya went down from 9 to 2.

BENGALI: 353725 out of 354188 tests passed. 463 failed (0.130722%)
DEVANAGARI: 707311 out of 707394 tests passed. 83 failed (0.0117332%)
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%)
MALAYALAM: 1048136 out of 1048334 tests passed. 198 failed (0.0188871%)
MYANMAR: 1115830 out of 1123883 tests passed. 8053 failed (0.716534%)
ORIYA: 42327 out of 42329 tests passed. 2 failed (0.00472489%)
SINHALA: 271596 out of 271847 tests passed. 251 failed (0.0923313%)
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/harfbuzz/harfbuzz/issues/1071

From the new code (first paragraph is from the OT Devanagari spec.):

  /*   o Reorder matras:
   *
   *     If a pre-base matra character had been reordered before applying basic
   *     features, the glyph can be moved closer to the main consonant based on
   *     whether half-forms had been formed. Actual position for the matra is
   *     defined as “after last standalone halant glyph, after initial matra
   *     position and before the main consonant”. If ZWJ or ZWNJ follow this
   *     halant, position is moved after it.
   *
   * IMPLEMENTATION NOTES:
   *
   * It looks like the last sentence is wrong.  Testing, with Windows 7 Uniscribe
   * and Devanagari shows that the behavior is best described as:
   *
   * "If ZWJ follows this halant, matra is NOT repositioned after this halant.
   *  If ZWNJ follows this halant, position is moved after it."
   *
   * Test case, with Adobe Devanagari or Nirmala UI:
   *
   *   U+091F,U+094D,U+200C,U+092F,U+093F
   *   (Matra moves to the middle, after ZWNJ.)
   *
   *   U+091F,U+094D,U+200D,U+092F,U+093F
   *   (Matra does NOT move, stays to the left.)

Fixes https://github.com/harfbuzz/harfbuzz/issues/1070

Test case added with Adobe Devanagari.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1073

Apparently this changed between Win 7 and Win 10.

Fixes https://github.com/harfbuzz/harfbuzz/issues/967

To be continued in https://github.com/harfbuzz/harfbuzz/issues/1044

Towards possibly using Null pool for some nil objects.

Fixes #779 and follow up to b01d9b3d

No numbers changed.

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