bug#43405: Tool bar item doesn't align to the right edge

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 43405@debbugs.gnu.org,  juri@linkov.net
> Date: Tue, 22 Sep 2020 17:14:18 +0200
> 
> No, I just want to be able to produce:
> 
> This is text                          This is right justified
> 
> And have the "This is right justified" stay right justified as the
> window size and line content changes (unless the user deletes that
> stretch glyph in the middle).
> 
> If the line became longer than could be displayed in the available
> window width, then I think the stretch glyph in the middle would just
> be treated as a single space (and hence line continuation/truncation
> would work as normal).

Then the way to do this is to set some flag on the first glyph after
the stretch, lay out the glyphs on the screen line as usual, then go
back to that marked glyph and recompute the width of the stretch.  It
will complicate the likes of display_line a bit, and will need to
disable some redisplay optimizations (so there should be some easy way
of knowing that such lines are in the buffer).