That function checks for buffer->successful already.  No need
to check at call site.

Newer compilers / language allows structs with constructor in union.
So, this was not actually testing anything.  Indeed, the recent
change in DISALLOW_COPY *is* making some of our types non-POD.
That broke some bots.

Just remove this since it wasn't doing much, and I'd rather have
DISALLOW_COPY.

Show up with gcc -O0.

There's a few more but those are functions that need to be filled in.

Maybe this is a lost battle...

No logic change.

Based on experimenting with Uniscribe to extract grammar and categories.

Failures down from 44 to 35:

KHMER: 299089 out of 299124 tests passed. 35 failed (0.0117008%)

We still don't enforce the one-matra rule pre-decomposition, but enforce
an order and one-matra-per-position post-decomposition.

https://github.com/harfbuzz/harfbuzz/issues/667

Alternative woul have been to resurrect F_COMBINE that I removed in
70136a78

But this does it for now.  I'm not sure why check-static-inits.sh didn't
catch this before.  Clang -Weverything bot did:

  CXX      libharfbuzz_la-hb-ot-shape-complex-indic.lo
hb-ot-shape-complex-indic.cc:99:1: warning: declaration requires a global constructor [-Wglobal-constructors]
indic_features[] =
^
1 warning generated.
  CXX      libharfbuzz_la-hb-ot-shape-complex-khmer.lo
hb-ot-shape-complex-khmer.cc:36:1: warning: declaration requires a global constructor [-Wglobal-constructors]
khmer_features[] =
^
1 warning generated.

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.

Probably not what Uniscribe does, but good idea?

This makes test suite happy again (at 44) while fixing the sequences
we were fixing, which were the following with KhmerUI.ttf:

  U+1789,U+17BC
  U+1789,U+17D2,U+1789
  U+1789,U+17D2,U+1789,U+17BC

Fixes rest of https://github.com/harfbuzz/harfbuzz/issues/974

Part of https://github.com/harfbuzz/harfbuzz/issues/974

Khmer spec has only one reordering phase, and only simple prebase
matra and Coeng-Ro reordering.  Implement that.  Specifically,
this was done to address recognizing different orders of the matra
and Coeng-Ro sequence.  That said, some combinations are now
reordered differently from Uniscribe.  Not clear if that's intended
or a bug in Uniscribe.  The following two sequences render the same
in Uniscribe whereas we reorder them differently:

  U+17A0,U+17D2,U+179A,U+17C2
  U+17A0,U+17C2,U+17D2,U+179A

For that reason, our test suite numbers regressed slightly.  Used
to be at 34 for fails, now at:

KHMER: 299080 out of 299124 tests passed. 44 failed (0.0147096%)

But generally a good change, and removed lots of code.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1026

Indic shaper already had this check.  We removed it when forking
Khmer shaper by mistake.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1464623

Test suite results are unchanged (34).

Towards possibly using Null pool for some nil objects.

New numbers (down from 44):

KHMER: 299085 out of 299124 tests passed. 39 failed (0.0130381%)

Used by Sinhala only.

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%)