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

Related
https://github.com/harfbuzz/harfbuzz/issues/345
https://github.com/harfbuzz/harfbuzz/pull/629

SVG is unused. Just keeping it here.

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

Might fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1463

Looks like hb-ot-font is complete after all!

CFF font names and CFF glyph metrics don't work though...

https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern

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

Hopefully fully fixes
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1856

This reverts commit 5daf3bd4.

If other atexit callbacks try to destruct the objects we destruct
in atexit callbacks, bad things will happen.

I'll come up with some other way to catch premature destruction
of HB-owned objects.

Fixes https://github.com/behdad/harfbuzz/issues/618

After 4a27c17e we do not need
the special GlyphID.cmp(hb_codepoint_t), so just make GlyphID
a typedef.

Fixes https://github.com/behdad/harfbuzz/issues/612