[HarfBuzz] Inconsistent shaping across systems

[Simon Cozens]

More details at https://github.com/simoncozens/sile/issues/164 , but in
a nutshell, using Noto Sans CJK JP and TTB direction, Ubuntu 12 gives:

$ md5sum  ~/.fonts/NotoSansCJKjp-Light.otf
f269c193b1ef0b739030d56386670c3f  /home/simon/.fonts/NotoSansCJKjp-Light.otf
$ hb-shape --direction=ttb ~/.fonts/NotoSansCJKjp-Light.otf '世界'
[gid9861=0 at -500,-880+0,-1000|gid27506=1 at -500,-880+0,-1000]

and OS X (not using CoreText) gives

$ md5sum ~/Library/Fonts/NotoSansCJKjp-Light.otf
f269c193b1ef0b739030d56386670c3f  ~/Library/Fonts/NotoSansCJKjp-Light.otf
$ hb-shape --direction=ttb ~/Library/Fonts/NotoSansCJKjp-Light.otf '世界'
[gid9861=0 at -500,-894+0,-1000|gid27506=1 at -500,-894+0,-1000]