Re: master c22b735: (string-pixel-width): Rewrite to avoid side effects

[Eli Zaretskii]

>     * src/xdisp.c (Fwindow_text_pixel_size): Allow `window` to be a buffer.
>     * lisp/emacs-lisp/subr-x.el (string-pixel-width): Simplify accordingly.

Bother: this doesn't make sure w->contents is the current buffer,
which I think many display routines assume without checking (or barf
if it isn't true).  So I think this is unsafe.