bug#64971: 30.0.50; string-pixel-width return value is affected by 'line-prefix

[Ihor Radchenko]

According to the docstring of `string-pixel-width', it "Return the width
of STRING in pixels."

Consider the following:

(string-pixel-width "asd") ;=> 30
(string-pixel-width (propertize "asd" 'line-prefix "123")) ;=> 60

Should the value of 'line-prefix be appended to the string width?

According to https://debbugs.gnu.org/cgi/bugreport.cgi?bug=64696#173,
'line-prefix "does not affect the width of the string itself".

In GNU Emacs 30.0.50 (build 5, x86_64-pc-linux-gnu, GTK+ Version
 3.24.38, cairo version 1.17.8) of 2023-07-29 built on localhost
Repository revision: ce48073f1597ceecb82800e71c89b53badc9f9d0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
System Description: Gentoo Linux

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Ihor Radchenko // yantar92,
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