This makes defining HB_NDEBUG much less relevant, to the
point of irrelevance.  Sorry about all the fuss in previous
release!

API changes:
- If NDEBUG is defined, define HB_NDEBUG
- Disable costlier sanity checks if HB_NDEBUG is defined.

In 1.2.3 introduced some code to disable costly sanity checks if
NDEBUG is defined.  NDEBUG, however, disables all assert()s as
well.  With HB_NDEBUG, one can disable costlier checks but keep
assert()s.

I'll probably add a way to define HB_NDEBUG automatically in
release tarballs.  But for now, production systems that do NOT
define NDEBUG, are encouraged to define HB_NDEBUG for our build.

Saves some sweet time and binary size!

Exercises fix in c335fd79

See discussion:
https://lists.freedesktop.org/archives/harfbuzz/2016-February/005489.html

This file is rather obsolete.  Still, give it a refresh.

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.