Simplify redisplay-x/separate_textual_runs

[Aidan Kehoe]

 Ar an triochadú lá de mí Márta, scríobh Olivier Galibert: 

 > separate_textual_runs was a nice, simple function ages ago, but not
 > anymore.  Now it's a not very readable ifdef mess.  Building 4
 > functions for the mule/nomule, xft/xlib cases allows for drastic
 > simplifications for most of them.
 > 
 > 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.

[...]
 > @@ -297,11 +335,54 @@
 >        run_storage[runs_so_far - 1].len =
 >  	text_storage - run_storage[runs_so_far - 1].ptr;
 >        /* Dimension retains the relevant value for the run before it. */
 > -      if (2 == dimension)
 > +      if (dimension == 2)
 >  	run_storage[runs_so_far - 1].len >>= 1;
 >      }

For C, the CONSTANT == VARIABLE idiom has a clear advantage over the other
way around (that is, ‘2 = dimension’ doesn’t compile) and the more it’s
used, the more readable it gets. Please don’t change this idiom in future.

-- 
On the quay of the little Black Sea port, where the rescued pair came once
more into contact with civilization, Dobrinton was bitten by a dog which was
assumed to be mad, though it may only have been indiscriminating. (Saki)