[HarfBuzz] Refactoring mark skipping

[Khaled Hosny]

On Mon, Jan 16, 2012 at 10:22:40PM -0500, Behdad Esfahbod wrote:
> Hi Jonathan,
> 
> I refactored mark skipping all in one place and pushed to master.  Can you
> please review?  Also interested to see test results in Firefox.

Not sure if this is related, but now it seems some OpenType rules are not
triggered for the first and last last glyphs in a run, for example,
using Scheherazade 'لا' gives plain initial and final glyphs, while ' لا '
give the correct contextual forms:

  $ hb-shape --no-positions --no-clusters ScheherazadeRegOT.ttf 'لا'
  <uni0627.fina|uni0644.init>

  $ hb-shape --no-positions --no-clusters ScheherazadeRegOT.ttf ' لا '
  <space|uni0627.fina.postLamIni|uni0644.init.preAlef|space>

This was fine last week.

Regards,
 Khaled