Part of https://github.com/harfbuzz/harfbuzz/issues/1652

Ugliness was fixed in 30eab97a
But yeah, my smell detector was working.  Ugliness was buggy.

Buffer might be relocated inside replace_glyphs().

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=895117

Show up with gcc -O0.

There's a few more but those are functions that need to be filled in.

Maybe this is a lost battle...

Even without FT or OT font funcs implementing get_nominal_glyphs(), there's measurable
speedup.

Even has measurable speedup...

Towards separating the common case into its own loop.

https://github.com/harfbuzz/harfbuzz/issues/653#issuecomment-423905920

https://github.com/harfbuzz/harfbuzz/commit/62d1e0852a5549a1b510ad46a4b89f12730bb708#commitcomment-30613091

Fixes https://github.com/harfbuzz/harfbuzz/issues/653

Sorry for the noise, downstream custom builders.  Please adjust.

Towards possibly using Null pool for some nil objects.

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

Fixes https://github.com/behdad/harfbuzz/issues/509

We are going to implement Unicode Arabic Mark Ordering Algorithm:

  http://www.unicode.org/reports/tr53/tr53-1.pdf

which will reorder marks out of their sorted ccc order.  Adjust
normalizer to stop combining as soon as dangerous ordering is
detected.

Fixes https://github.com/behdad/harfbuzz/issues/411

New API:
- hb_font_get_nominal_glyph_func_t
- hb_font_get_variation_glyph_func_t
- hb_font_funcs_set_nominal_glyph_func()
- hb_font_funcs_set_variation_glyph_func()
- hb_font_get_nominal_glyph()
- hb_font_get_variation_glyph()

Deprecated API:
- hb_font_get_glyph_func_t
- hb_font_funcs_set_glyph_func()

Clients that implement their own font-funcs are encouraged to replace
their get_glyph() implementation with a get_nominal_glyph() and
get_variation_glyph() pair.  The variation version can assume that
variation_selector argument is not zero.

Fixes assert fail in https://github.com/behdad/harfbuzz/issues/161
with libharfbuzz-fuzzing.

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

Test passes now.

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.

Fixes https://github.com/behdad/harfbuzz/issues/152

Part of https://github.com/behdad/harfbuzz/issues/136

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=541608
and cluster test.

Needed for upcoming merge-clusters fix.

Fixes https://github.com/behdad/harfbuzz/pull/124