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: Mon, 21 Sep 2020 20:30:53 +0200
> 
> So let's assume we do this by exending the display spec to allow
> 
> '(:right-justify t)
> 
> which would mean to move everything on this line as far to the right
> in the window as possible.
> 
> At some point weʼd end up in 'gui_produce_glyphs' with a 'struct
> iter' pointing at the char with that property set. Then:
> 
> remember it->current_x
> loop over the iters until we hit eol or max_x, calling PRODUCE_GLPYHS
> The final it->current_x minus the remembered one is the width of the remaining
> glyphs on the line.
> Now set it->current_x to the window right edge minus the width.
> 
> Does that sound like it would work? Is there a more direct way of
> calculating that width? (I got lost in all the various move_to
> functions).

I'd first try to repeat what we do for :align-to support: insert a
stretch glyph of a suitable width, computed using it->last_visible_x
and the width of the image for the button that has to be
right-justified.  See produce_stretch_glyph (except that most of it is
not relevant, since :align-to supports a lot of functionalities).