Fixes https://github.com/behdad/harfbuzz/issues/187
Funcs implementations that have a non-zero horizontal origin must
implement the glyph_h_origin() callback, nothing new here.
Other implementations (all I know of!) can simply not set
glyph_h_origin() now. I did that for hb-ot and hb-ft in
44f8275080, though that broke the
fallback shaper because the default was returning false...