[HarfBuzz] behavior of mark-width-zeroing

[Jonathan Kew]

Hi Behdad,

I've opened https://github.com/behdad/harfbuzz/pull/43 with the patches 
I'm proposing to fix the legacy Thai case. However, it looks like my 
lack of git-fu means that the PR includes a couple of earlier commits as 
well; apparently I didn't rebase correctly. Sorry about that!

Anyhow, the relevant commits for the Thai case are
   6425ecfc98278dc671c4a6a7ccceb594e3714bde
   2de09a10dcb1eed1ee83fa67098d375292a7bcb6

JK