bug#12745: crash in bidi_pop_it during (idle) redisplay

[Alp Aker]

> Couldn't you simply put the $ (or whatever) in the display string,
> where you now put the u+e000 character?

Probably.  IIRC I found that (a) I needed to put a cursor property on
the overlay string to get sensible cursor movement at the end of
lines, but (b) putting the cursor property on a string that gets
displayed as a stretch glyph via its display property didn't work.  So
I needed to have a blank character there to put a cursor property on,
even when the newline has a nil display table entry.  At that point it
made the code simpler to use that char for visible newline display as
well, via display table manipulation.

But if this turns out to cause too many headaches for the redisplay
routine I can rewrite it along the lines you describe.