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?
>
> 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.

In other words, my earlier explanation wasn't accurate (it's been a
while since I thought about this code).  The blank character in the
overlay string is there for the sake of the cursor property.  That
it's a weird Private Use Area character instead of a space is just to
simplify the whitespace-mode compatibility hack.