Make nil glyph_h_origin() function return true

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
44f82750807475aa5b16099ccccd917d488df703, though that broke the
fallback shaper because the default was returning false...