Simplify redisplay-x/separate_textual_runs

[Stephen J. Turnbull]

 > Aidan writes:

 > > You've just done that Unicode conversion to throw away the result.

*sigh*  You're right, but it took minutes to figure out what you were
talking about.  Please be more explicit.

Olivier Galibert writes:

 > Ok, I don't see how.  Did you miss the '~' ?

No, the problem is that from Aidan's comment on down, ucs is dead.
Look here:

 > > +      *(XftChar16 *)text_storage = str[i];

s/str[i]/ucs/ and win (tested).  Which is obvious, but only if you
don't have to fight through a thicket of #ifdefs that you're trying to
disentangle.  That the fix is this easy is proof of your concept, IMO.