Minor
[graphite2] Don't crash if language is not set

https://bugs.webkit.org/show_bug.cgi?id=113796
Bug 63107 - FTBFS on ppc64: symbols marked with 'D' on powerpc64 instead
[Arabic] Zero marks by GDEF, not Unicode category

Testing shows that this is closer to what Uniscribe does.

Reported by Khaled Hosny:

"""
commit 568000274c8edb5f41bc4f876ce21fcc8bdaeed8
...
This commit is causing a regression with Amiri, the string “هَٰذ” with
Uniscribe and HarfBuzz before this commit, gives:

	[uni0630.fina=3+965|uni0670.medi=0+600|uni064E=0@-256,0+0|uni0647.init=0+926]

But now it gives:

	[uni0630.fina=3+965|uni0670.medi=0+0|uni064E=0@-256,0+0|uni0647.init=0+926]

i.e. uni0670.medi is zeroed though it has a base glyph GDEF class.
"""

The test case is U+0647,U+064E,U+0670,U+0630 with Amiri.
Put back MemoryBarrier fallback implementation on MINGW32

This almost reverts 2761e8a632c14353f286708898be8df6ebad7407,
but only if under MINGW32, so it doesn't affect MSVC.
Don't compose Hangul jamo

See thread "an issue regarding discrepancy between Korean and Unicode
standards" on the mailing list for the rationale.  In short: Uniscribe
doesn't, so fonts are designed to work without it.
Add note re Hangul shaping
[git.mk] Ignore ar-lib
0.9.15
Minor
[git.mk] Update
[old] Speed up Unicode properties access in hb-old shaper

Just to be sure it's not punishing the old shaper performance
numbers.  Doesn't seem to have a measurable effect.
[util] Add --num-iterations

Useful for profiling shapers.
Fix typo in unused macro