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

[Robert Pluim]

>>>>> On Fri, 18 Sep 2020 11:58:21 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> OK, so I took a look, and Iʼm not sure itʼs possible with the native
    >> tool bar. We have '(space :align-to right)', but that just inserts
    >> space up to a specified location, everything subsequent is
    >> appended. In order to calculate the correct location, Iʼd need to know
    >> the width of everything that came after the space, which only
    >> redisplay can tell us, unless thereʼs a function Iʼve missed?

    Eli> The support for doing this with the native tool bar must be in C, and
    Eli> should indeed be part of the display engine.  So everything redisplay
    Eli> knows should be at your fingertips.

Your fingertips maybe, not mine :-)

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).

Robert