bug#37858: 27.0.50; Ensure a minimum width for `space` display prop

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Mon, 21 Oct 2019 16:03:58 -0400
> 
> So, I'd like to extend our `space` specifications so as to be able to
> specify a minimum width.  I came up with the patch below which lets you
> write:
> 
>     (space :align-to FOO :min-width BAR)
> 
> which seems to work fine, but while trying to update the Elisp doc for
> it I realized that maybe a better option is to extend the acceptable
> forms for FOO so it can be of the form:
> 
>     (space :align-to (max FOO (+ BAR current-x)))

Since :align-to already supports expressions of the forms described in
the node "Pixel Specification", to have the latter you'd need:

  . implement a new OP called 'max' (and probably also 'min' for a
    good measure);
  . implement a new POS called, say, 'beg', which will evaluate to the
    x coordinate of where the space display property begins

and then you will have your feature for free, I think.