Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

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

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

Apparently calibri.ttf does this:
https://github.com/fonttools/fonttools/pull/1094#discussion_r148933791

Seems to work.  Yay!

Still to do: run kerning if GPOS doesn't have 'kern' feature.

Almost there.

Pushing this out early to catch bot errors since I'm using template tricks
we were not using in HarfBuzz before.

get_glyph_from_name() coming soon.

Such that client cannot accidentally destroy them, even though that
will be a bug in their code...

The get_glyph_from_name() is incorrect; needs fixing.

Turns out we already have support for “post” table, it just needed to be
activated and put in use.

Fixes https://github.com/behdad/harfbuzz/issues/474

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1452#c5

CC https://github.com/behdad/harfbuzz/issues/139

per https://github.com/behdad/harfbuzz/issues/418#issuecomment-280873811