I swear I have no idea why this is needed, but it is...

Used to be ascent+descent.  Whatever. This is more convenient for
upcoming changes.

New API:
+hb_face_collect_unicodes
+hb_face_collect_variation_selectors
+hb_face_collect_variation_unicodes

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

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