Simplify redisplay-x/separate_textual_runs

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > We don’t have a good record on this, 

Not since Martin and Hrvoje left, no.

 > and doing something to improve performance is preferable to the
 > tuits approach.

Sure.  However, profiling helps identify which function to
optimize. It does not tell you much about the optimizations that will
be effective, unless you put a large amount of effort into the
profiling itself.

 > My last changes to separate_textual_runs eliminated unnecessary
 > assignments during the loop that had been there since the Mule
 > merge.

But AIUI, optimization by the compiler will do that for you, too.  The
resulting code is usually cleaner and more readable, but AFAIK it has
no impact on performance in an optimized build.  Is that wrong?