bug#5977: 24.0.50; Lao HELLO is incorrectly displayed

[Eli Zaretskii <eliz@gnu.org>]

> From: Jason Rumney <jasonr@gnu.org>
> Date: Tue, 20 Apr 2010 07:15:49 +0800
> Cc: 5977@debbugs.gnu.org
> 
> Peter Dyballa <Peter_Dyballa@Freenet.DE> writes:
> 
> > The Lao greetings are incorrectly rendered:
> 
> This seems to be the case with all the scripts that use otf to display
> since the bidi code was merged.

Are you saying that you don't see similar problems on MS-Windows?
Because I do.

The Lao script does not use any R2L characters (AFAICS), so the only
issue with the bidi code here is character compositions.  Other than
that, the characters are not reordered, so cursor motion should be
strictly left to right, as usual (although jumps are expected, due to
problems with composed characters).  All this code is in terminal
independent parts of the display engine, so it should affect all GUI
ports in the same way.  The only difference between Uniscribe and
libotf should be in how they render un-composed characters, such as
u+0EB5, that the bidi redisplay incorrectly puts on the screen.

Somebody who knows how character compositions work in Emacs, please
help me to DTRT with them in the bidi display!