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.

Not sure why the FT functions were returning advance 1024.  This
caused failure on drone.io.  Switch to hb-ot-font and disable
glyph names.

That commit moved the advance adjustment for mark positioning to
be applied immediately, instead of doing late before.  This breaks
if mark advances are zeroed late, like in Arabic.  Also, easier to
hit it in RTL scripts since a single mark with non-zero advance is
enough to hit the bug, whereas in LTR, at least two marks are needed.

This reopens https://github.com/behdad/harfbuzz/issues/211
The cursive+mark interaction is broken again.  To be fixed in a
different way.

Apparently I broke this 86c68c7a.
Fix coming.

They are unused currently.  We can add later if we hook them up
to anything useful.

This better emulates Unicode grapheme clusters.

Note that Uniscribe does NOT do this, but should be harmless with most clients,
and improve fallback with clients that use HarfBuzz cluster as unit of fallback.

Fixes https://github.com/behdad/harfbuzz/issues/217

Fixes https://github.com/libass/libass/issues/212

Test from https://github.com/behdad/harfbuzz/issues/223

I forgot that we do run hb-fuzzer on tests in shaping/tests/fuzzed.tests.

Fixes https://github.com/behdad/harfbuzz/issues/223

Right now we cannot test this because it has to be tested using hb-fuzzer.
We should move all fuzzing tests from test/shaping/tests/fuzzed.tests to
test/fuzzing/ and have its own test runner.  At that point, should add
test from this issue as well.

Not going to do with util/, but is convenient here.

Fixes https://github.com/behdad/harfbuzz/issues/227

Fixes https://github.com/behdad/harfbuzz/issues/169

This is what Microsoft's implementation does.  Marks that need advance
need to add it back using 'dist' or other feature in GPOS.  Update tests to
match.

Fixes https://github.com/behdad/harfbuzz/issues/211

What happens in that bug is that a mark is attached to base first,
then a second mark is cursive-chained to the first mark.  This only
"works" because it's in the Indic shaper where mark advances are
not zeroed.

Before, we didn't allow cursive to run on marks at all.  Fix that.
We also where updating mark major offsets at the end of GPOS, such
that changes in advance of base will not change the mark attachment
position.  That was superior to the alternative (which is what Uniscribe
does BTW), but made it hard to apply cursive to the mark after it
was positioned.  We could track major-direction offset changes and
apply that to cursive in the post process, but that's a much trickier
thing to do than the fix here, which is to immediately apply the
major-direction advance-width offsets...  Ie.:

https://github.com/behdad/harfbuzz/issues/211#issuecomment-183194739

If this breaks any fonts, the font should be fixed to do mark attachment
after all the advances are set up first (kerning, etc).

Finally, this, still doesn't make us match Uniscribe, for I explained
in that bug.  Looks like Uniscribe applies minor-direction cursive
adjustment immediate as well.  We don't, and we like it our way, at
least for now.  Eg. the sequence in the test case does this:

- The first subscript attaches with mark-to-base, moving in x only,
- The second subscript attaches with cursive attachment to first subscript
  moving in x only,
- A final context rule moves the first subscript up by 104 units.

The way we do, the final shift-up, also shifts up the second subscript
mark because it's cursively-attached.  Uniscribe doesn't.  We get:

[ttaorya=0+1307|casubscriptorya=0@-242,104+-231|casubscriptnarroworya=0@20,104+507]

while Uniscribe gets:

[ttaorya=0+1307|casubscriptorya=0@-242,104+-211|casubscriptnarroworya=0+487]

note the different y-offset of the last glyph.  In our view, after cursive,
things move together, period.

No effect.

Part of fixing https://github.com/behdad/harfbuzz/issues/211

Right now the position_finish_advances() is empty.  To be used for
spacing attachments proposal later.