bug#11073: 24.0.94; BIDI-related crash in redisplay with certain byte sequences

[Eli Zaretskii]

> From: Kenichi Handa <handa@m17n.org>
> Cc: eliz@gnu.org, 11073@debbugs.gnu.org
> Date: Fri, 06 Apr 2012 10:13:12 +0900
> 
> > makes the behavior of some macros subtly more complex/different and
> > hence adds a nasty complexity.
> 
> That's mostly because I didn't write a proper comments on
> the relavant macros, and didn't provide a better macros for
> such a case as Eli's.

I added comments to the relevant macros (as trunk revision 107781) to
warn about these subtleties.