I was overloading ApplyDXArray() with a HarfBuzz specific implementation
because the GenericSalLayout one was screwing right to left kerning, but
it seems to have broken left to right full justifications. Since
mnXOffset was introduced a bit earlier to fix a similar issue, it can
now be used here as well to minimize the possible side effects.

Seems to work fine for both left to right and right to left text now,
but at least one of my Arabic tests is regressing, so might need some
tweaking.

Change-Id: I1239b0ec77a4978f981a480400a6d01cda18af79