Fix for bug 8695466 GPOS combining mark positioning broken before space

This patch makes segmentation into script runs behave the same in RTL
as in LTR modes - so that inherited script characters are always
associated with the preceding run. Otherwise, for a sequence such as
u+0631 u+064d u+0020, it would get split after the first character,
which would lose the ability to correctly position the u+064d mark.

Change-Id: I3c12ba1b77d18334f55e707f518be1046e6b339b