bug#69992: Minor improvement to image map transformation logic

[David Ponce]

[...]
> The attached new patch fixes the logic to be consistent with Emacs
> internal implementation, plus some other tweaks to check if a
> transformation apply before to call the transformation function.
> I also updated some tests according to functions changes.
> Here is a possible change log:
>
> 2024-03-28  David Ponce  <da_vid@orange.fr>
>
>      * lisp/image.el (image--compute-scaling)
>      (image--compute-rotation): New functions.
>      (image--compute-map, image--compute-original-map): Use them.
>      Ensure all transformations are applied or undone according to what
>      Emacs does internally.  Call a transformation function only when
>      needed.  Fix doc string.
>      (image--scale-map): Assume effective scale argument.
>      (image--rotate-map): Assume effective rotation argument.
>      (image--rotate-coord): Improve doc string.
>      (image--flip-map): Remove no more used flip argument.
>
>      * test/lisp/image-tests.el (image-create-image-with-map): Use a
>      valid SVG image otherwise `image-size' will not return a valid
>      value and calculation of scale could fail.
>      (image-transform-map): Update according to changed signature of
>      image--flip-map.
[...]

Hello,

Please find attached a new patch with an additional small fix I forgot to
include.  Sorry.

Regards

image-compute-map-V1.patch
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