Fixes https://github.com/harfbuzz/harfbuzz/issues/635

FT_Set_Var_Blend_Coordinates() is not available when "Multiple Master
font interface" is disabled in freetype's modules.cfg

Ouch!

Add tests.

Fixes https://github.com/harfbuzz/harfbuzz/issues/628

New API:
hb_coretext_font_create()

It's as good as it gets, and seems to be on par with previous set implementation
in my benchmark.

Would be great if someone can double-check my bitops.

With new set implementation, this became really costy.  Optimize it.
There's more to be done, but this shaves off most of the fat.

Part of fixing https://bugs.chromium.org/p/chromium/issues/detail?id=782220

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