Fixes https://github.com/behdad/harfbuzz/issues/505

* Minor

* [coretext] Fix leak

* [coretext] Do not reset num_features

* [coretext] allow to disable kern; re-enabling doesn't seem to be working

Fixes https://github.com/behdad/harfbuzz/issues/376

Part of https://github.com/behdad/harfbuzz/issues/376
Also see https://github.com/roozbehp/unicode-data/issues/6

Test added, using NotoSansCham built from Noto Phase III sources.

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%)

Not used yet.

* Shaping tests for Tibetan vowels

* Test-cases for the Dzongkha contractions with multiple vowel-signs added.

* going to be removed

* Extended contraction-test-cases to all test cases in contractions.txt that actually use multiple-vowels (113 cases)

* Undone change for Tibetan vowel u

* removed comment on reordering that became invalid with roll-back

* Support for Dzongkha contractions with multiple vowel-signs

* Removed non-functional and unnecessary defines for HB_MODIFIED_COMBINING_CLASS_CCC138,140

Add Python scripts to generate the full win32/config.h.win32 and
src/hb-version.h which can be used to build directly from a GIT
checkout.  Since the scripts are currently intended for building from a
GIT checkout, these are not distributed in the release tarballs.

Also, support the re-build of Ragel-generated .hh headers using the NMake
build system, and allow one to specify the path of the Ragel executable
if a suitable one cannot be found in the PATH.

Update the Win32/NMake build documentation to let people know about how
these mechanisms can be utilized.

If an application provides a malloc replacement through
hb_malloc_impl() it is important that it is used to allocate
everything, but the use of strdup() circumvents this and
causes system malloc() to be called instead. This pairs
badly with the custom hb_free_impl() being called later.

Hide them like Mongolian Free Variation Selectors instead.

Fixes https://github.com/behdad/harfbuzz/issues/463

clang's new -Wcomma compiler option warns about possible misuse of the
comma operator such as between two statements.

hb-common.cc:190:9 [-Wcomma] possible misuse of comma operator here
hb-ot-layout-gsubgpos-private.hh:345:30 [-Wcomma] possible misuse of
comma operator here
hb-shape-plan.cc:438:26 [-Wcomma] possible misuse of comma operator here

https://github.com/behdad/harfbuzz/issues/455

There are more broken versions of Tahoma out there on various Windows releases,
so we need to add them to our blacklist to avoid broken rendering.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1279925 for details.

Fix the check conditions in config-msvc.mak and info-msvc.mak so that
the ICU items does indeed get built into the HarfBuzz main DLL, and that
the correct configuration info is displayed.

Also update the checks in detectenv-msvc.mak so that we can detect that
we are using Visual Studio 2017 (although the 2015-built binaries use
the same CRT DLL as the 2017 ones).

See https://bugzilla.mozilla.org/show_bug.cgi?id=1336600

Current fonttools (3.9.1) generate subset-file called font.subset.ttf instead of older font.ttf.subset