Simplify redisplay-x/separate_textual_runs

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > Thanks! Please profile the two (with etc/HELLO at least working for the CJK
 > languages) and post comparisons, separate_textual_runs is called quite a
 > lot.

With all due respect, I don't think this is very useful at this point.

First, in light usage I perceive no slowdown, and don't expect a
significant speedup, either.

Second, before optimizing separate_textual_runs, which has always been
a one-pass O(text length) algorithm, and thus hard to beat (or screw
up), redisplay (and therefore separate_textual_runs, I suspect) seems
to be called multiple times for an awful lot of changes; saving whole
calls the separate_textual_runs would be more helpful.

There's also an issue that Xft fonts don't seem to be properly cached
(although on the server side it may not be implemented, which is a
fact for PROBLEMS, and the chatter I'm seeing from Xft about
initializing fonts may be due to me putting the debug printfs in the
wrong place).