bug#47712: 27.1; Provide `string-display-width` function, which takes properties into account, `substring-width`

[Eli Zaretskii]

> Date: Mon, 12 Apr 2021 17:15:17 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 47712@debbugs.gnu.org
> 
> > But for we can still look at the micro benchmark.  The `string-width` 
> > function is 200 times faster than the `string-pixel-width` function. 
> 
> And if you reuse the same temp buffer?

My benchmarking indicates that reusing the buffer makes
string-pixel-width only 10 times slower than string-width, i.e. 50
about microseconds per call.