bug#9571: 24.0.50; user option to turn off bidi, please

[Stefan Monnier]

> IOW, I feel that the issue is blown out of proportions for reason I
> cannot understand.  By and large, THERE IS NO PROBLEM.  Fear of
> problems, maybe.  But no real problems, at least not reported ones.

Agreed.

There have been a fair number of behavior bugs with respect to cursor
positioning (that's the tricky part of your changes, and IIUC the only
part that can't just be turned off by bidi-display-reordering), but they
seem to be mostly under control now (new cursor bugs are just as
likely now to exist in Emacs-23 as well).

The other source of problem has been performance, and AFAICT it's always
been linked to bidi-paragraph-direction (unsurprisingly: the rest of
the changes have almost no impact on algorithmic complexity, and even
the potentially large slowdown on the core "step to next position"
operation is drowned in all the rest of the work we do per-position
anyway).  We can make that default to `left-to-right' if needed, but
hopefully that won't even be necessary.


        Stefan