Previously we made CGJ unskippable.  Now, if CGJ did NOT prevent
any reordering, allow skipping over it.  To make this work we
had to make changes to the Arabic mark reordering algorithm
implementation to renumber moved MCM marks.  See comments.

Fixes https://github.com/harfbuzz/harfbuzz/issues/554

Ouch!

Add tests.

From https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4273

Particularly hazardous if the second layer mixes forward and backward lookups.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4336

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=787284

This only counts recursions right now.  Good start.

Hopefully...
Fixes https://github.com/behdad/harfbuzz/issues/429

At some point we should run tests under valgrind...

If offset is zero, we return Null() object.  Wasn't prepared for that.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4088

We were using ft funcs because these are CFF fonts, but we can avoid
glyph names instead and keep testing both ot and ft.

See https://github.com/behdad/harfbuzz/pull/590#issuecomment-341194673

We don't need to test the actual flag values.  For unsafe-to-break,
the --verify in run-tests.sh checks what we are interested in.

See https://github.com/behdad/harfbuzz/pull/590#issuecomment-341194673.

I simply removed the composite glyph and use the referenced simple
glyph directly.

This reverts commit 70137e2b.

Freetype and OT font functions give different positions for some glyphs
in this font (OT seems to be correct), but that is not what we are
interested in in this test.

See https://github.com/behdad/harfbuzz/pull/590#issuecomment-341045223.

https://github.com/behdad/harfbuzz/pull/590#issuecomment-340967271

These are CFF fonts and ot functions don’t support CFF glyph names yet.
The next commit will run all tests with ot functions.

https://github.com/behdad/harfbuzz/commit/49a41dc75931cc2d2f7e74b7801f8cc327039e8e#commitcomment-25203194