New API:
HB_UNICODE_MAX

And use.

To be moved to hb-face.h later.

To be moved to hb-face later.

Don't use accelerator (almost).  Hooks up Format13 as well.

OpenType cmap table can use gid=0 to mean "not covered" to produce
more optimized cmap subtables.  Return false from get_nominal_glyph()
for those.  hb-ft already does this.

New API:
+hb_face_builder_create
+hb_face_builder_add_table

Add HB_USE_ATEXIT guard around free_static_C_locale missed by previous
commit.

Add HB_USE_ATEXIT macros around free_static function definitions to
avoid unused function compiler warnings/errors.

Users don't expect a new reference returned from a get() function.
Indeed, all users of that API I foud where NOT destroying the reference.
Just change the implementations to NOT return a reference.

This applies to the following APIs:
hb_unicode_funcs_get_default()
hb_glib_get_unicode_funcs()
hb_icu_get_unicode_funcs()

Fixes https://github.com/harfbuzz/harfbuzz/issues/1134

Fixes https://github.com/harfbuzz/harfbuzz/issues/1129

We've had the proper barriers already.  No need for more with every access.