Simplify redisplay-x/separate_textual_runs

[Stephen J. Turnbull]

Olivier Galibert writes:

 > Please review (it's hairy code, another pair of eyes is no luxury) and
 > apply if happy.  It's probably easier in that specific case to check
 > the result rather than the diff.

A careful review later, but first a cosmetic suggestion: reorder the
functions to go from simpler to more complex, probably

separate_textual_runs_nomule
separate_textual_runs_xft_nomule
separate_textual_runs_xft_mule
separate_textual_runs_mule

 > +  for (i = 0; i < len; i++)
 > +    *text_storage++ = str[i];

Did the no-Mule, no-Xft case get *too bald* for you so you had to
introduce some hair?<wink>