[Simplify redisplay-x/separate_textual_runs] Fix union build, shut up gcc

[Stephen J. Turnbull]

Olivier Galibert writes:

 > The only part I'm really doubtful about is the ucs->byte split in the
 > -xft+mule translate_to_ics2 case.  I don't even know how to have the
 > code pass through there.

I think Aidan is the expert on that; I think that stuff is from his
JIT charsets work.  I took a hack at explaining it in my second post
(with the header comments, especially for -xft+mule), but I'd want
Aidan's review on that.