bug#74725: 31.0.50; image-scaling-factor is ignored by create-image

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 7 Dec 2024 13:13:58 +0100
> From:  David Ponce via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> While working with images, I found what seems an issue to me with
> `create-image' which unconditionally set the :scale image property to
> 'default' when not specified, ignoring the value of the option
> `image-scaling-factor'.
> 
> Here is an illustration:
> 
> (let ((image-scaling-factor 1.0))
>     (image-size
>      (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg")))
>      t))
> => (63 . 63)
> 
> (let ((image-scaling-factor 2.0))
>     (image-size
>      (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg")))
>      t))
> => (63 . 63)
> 
> (image-size
>    (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg" :scale 1)))
>    t)
> => (48 . 48)
> 
> (image-size
>    (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg" :scale 2)))
>    t)
> => (96 . 96)
> 
> You can replace `image-size' with `insert-image' and observe the same.
> 
> Here is a simple patch which fix the issue for me:
> 
> diff --git a/lisp/image.el b/lisp/image.el
> index ce97eeb3ca1..2c1e865c336 100644
> --- a/lisp/image.el
> +++ b/lisp/image.el
> @@ -536,7 +536,9 @@ create-image
>                            file-or-data)
>                      (and (not (plist-get props :scale))
>                           ;; Add default scaling.
> -                        (list :scale 'default))
> +                        (list :scale (if (numberp image-scaling-factor)
> +                                         image-scaling-factor
> +                                       'default)))
>   	           props)))
>         ;; Add default smoothing.
>         (unless (plist-member props :transform-smoothing)

AFAIU, this is supposed to be taken care of in image.c.

Alan, any ideas why this doesn't seem to work?