[HarfBuzz] Testing out Jonathan's proposal to fix mixed-up Khmer fonts

[Mathieu Pellerin]

Jonathan, Behdad,

Just spent the last twenty minutes testing out the proposed patch
applied to Jonathan's test builds over here
(http://ftp.mozilla.org/pub/mozilla.org/firefox/try-builds/jkew@mozilla.com-4bdec69a038e/).

The results are good:
- Kh-Battambang works (which was the font that revealed this mess)
- Hanuman works again
- Looking at Jonathan's test page, things are looking very good. I can
only see one broken font, that is Mo5V56do.ttf (which is also broken
on pango anyways, isn't used online as well as barely so in real
world, and suspect it might be a font problem)

I've also made a bunch of pango vs harfbuzz (patched) screenshots
while visiting the top-10 Khmer language web sites. After closely
inspecting the shots, all things are rendering properly under the
patched harfbuzz. The rendering is not only regression-free, I've
actually spotted two places where we are improving from pango's
misplacements!

Seems like this patch has hit the mythical pot of gold at the end of
the rainbow ;)

Mathieu