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]

This reverts commit 69f9fbc4.

See https://github.com/behdad/harfbuzz/issues/347#issuecomment-268873401
Fixes https://github.com/behdad/harfbuzz/issues/347

As discovered by libFuzzer / Chromium fuzzing.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=659496
CC https://github.com/behdad/harfbuzz/issues/139

Fixes https://github.com/behdad/harfbuzz/issues/363

Fixes https://github.com/behdad/harfbuzz/issues/363

It is desirable to be able to build against older versions of glib.

fd7a245d changed the configure check to require glib > 2.38 for
G_TEST_DIST. Before that, version 2.16 was required, but in fact,
since aafe395a, G_PASTE is being used, which was introduced in 2.19.1.
And since 0ef179e2, hb-glib uses GBytes, which were introduced in
2.31.10.

2.19.1 is rather old, but 2.38 is rather new. For Firefox, building
against 2.22 is still supported, although we could probably get away
with bumping that to 2.28. Either way, GBytes is not available.

Arguably, if you build against a glib that doesn't support GBytes,
you're not going to use the hb_glib_blob_create function, so we hide
the function when building against such a glib.

As for G_TEST_DIST, when building against versions of glib that don't
support it, we can fallback to the previous behavior, which, AIUI, was
just making the test not work when building in a separate directory.

Fixes https://github.com/behdad/harfbuzz/pull/377

Yay!!!!

Shape-plan caching is not implemented.

In prep for more changes.

It doesn't always work, not part of FreeType, and we were not going
to expose it in the API anyway.  Can always be added later.

Not hooked up to shaper yet.

Thanks Werner!

Eg. https://ci.appveyor.com/project/behdad/harfbuzz/build/1.0.48/job/o9mnd33kcdeeg30r

hb-open-type-private.hh:103:29: error: static data member 'OT::Device::<anonymous union>::<anonymous struct>::static_size' in unnamed class [-fpermissive]

Will probably change.

Untested.

Needed to access GDEF for varStore.

Still not hooked up from GPOS.

Not hooked up to GDEF yet.

This reverts commit 2859f1c7174d6f7ae5dcf0db5411bc0182b21594.
We are going in a different direction.