bug#55207: 29.0.50; tab-bar-format-align-right only works correctly with monospace characters

[Juri Linkov]

> When a character using a variable pitch font or an image (I've only
> tested this with XBM images) is displayed on the tab bar after
> 'tab-bar-format-align-right', the resulting text ends before the end of
> the tab bar.
>
> Exact recipe (assuming the default font for the tab bar is variable pitch):
>
> 1. $ emacs -Q
> 2. M-: (setq tab-bar-format '(tab-bar-format-align-right tab-bar-format-tabs 
> tab-bar-separator))
> 3. M-x tab-bar-mode
>
> After following these steps, the initial tab ends before the end
> of the tab bar, and the gap grows with each additional tab.

Thanks for the bug report.  Indeed the property :align-to
in tab-bar-format-align-right fails to align to the proper width
when using a variable pitch font.  Unfortunately, I have no idea
how to fix this.