From https://github.com/behdad/harfbuzz/issues/161

Fixes https://github.com/behdad/harfbuzz/issues/161

https://github.com/behdad/harfbuzz/issues/161

https://github.com/behdad/harfbuzz/issues/139#issuecomment-154202645

The feature is enabled for any character in the Arabic shaper.
We should experiment with using it for Arabic subtending marks.
Though, that has a directionality problem as well, since those
are used with digits...

Fixes https://github.com/behdad/harfbuzz/issues/141

Fixes https://github.com/behdad/harfbuzz/issues/162

Unused currently.  To be used for Syriac stretch implementation.

https://github.com/behdad/harfbuzz/issues/141

This one is a hack, but should be ok.

I'm starting to really like how free these new scratch_flags are.

Hopefully makes clang build happy

Hopefully this wouldn't break any compiler.  There was some magic
moments to getting this to compile...

In anticipation of further changes.  No functional changes.

API change: all fonts created with hb_font_create() now inherit from
(ie have parent) hb_font_get_empty().

It makes the binary smaller AND faster.  Yumm!

Now that we have a buffer-wide scratch flags facility, use it to
optimize away a few passes.

Used to be 100.  If a tabe needs more than one or two edits, it's
probably completely bogus...

Might help with speeding up fuzzing for
https://github.com/behdad/harfbuzz/issues/157

Test passes now.

U+2011 is <noBreak> equivaent of U+2010, so we should do the fallback
for it.  Currently fails.

See discussion here:
https://github.com/behdad/harfbuzz/commit/81ef4f407d9c7bd98cf62cef951dc538b13442eb

There's no way to disable this fallback, but I don't think it would
be needed.  Let's hope for the best!

Fixes https://github.com/behdad/harfbuzz/issues/153

This resurrects the space fallback feature, after I disabled
the compatibility decomposition.  Now I can release HarfBuzz
again without breaking Pango!

It also remembers which space character it was, such that later
on we can approximate the width of this particular space
character.  That part is not implemented yet.

We normalize all GC=Zs chars except for U+1680 OGHA SPACE MARK,
which is better left alone.