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
44f8275080, though that broke the
fallback shaper because the default was returning false...

src/hb-font.cc

@@ -109,7 +109,7 @@ hb_font_get_glyph_h_origin_nil (hb_font_t *font HB_UNUSED,
 				void *user_data HB_UNUSED)
 {
   *x = *y = 0;
-  return false;
+  return true;
 }
 static hb_bool_t
 hb_font_get_glyph_h_origin_parent (hb_font_t *font,