Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

Removes volatile from fallback implementation.  That was handwavy
anyway.

If ever int is not 32bit, we'll fix this...

https://bugzilla.mozilla.org/show_bug.cgi?id=1476334

New API:
+hb_font_get_glyph_advances_for_direction

New API:
+hb_font_get_glyph_h_advances
+hb_font_get_glyph_v_advances

We would need to write:

  for (Iter<T> it (...); bool (t); t++)

instead of:

  for (Iter<T> it (...); t; t++)

But I think it's an improvement in code readability in the long term.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

New API:
+hb_font_funcs_set_glyph_h_advances_func
+hb_font_funcs_set_glyph_v_advances_func
+hb_font_get_glyph_h_advances
+hb_font_get_glyph_h_advances_func_t
+hb_font_get_glyph_v_advances
+hb_font_get_glyph_v_advances_func_t

Finally, unified!

The code was badly broken.  In better shape now, but still, needs
a full review before ever working.

Also remove "    " null data for Tag.  Just use zeroes.

Most unused.

Misc. table loading now properly automated.

Faster / more compact code.