[ImageCache] don't convert CYMK image to RGB

[troopa81]

This PR is related to CMYK QEP

CMYK image support has been added in Qt 6.8.0. For now, it's not possible to render in a CMYK image, and as a consequence to scale it. So, QGIS will report a warning message to user if requested size is different from the CMYK image one.

Funded by Métropôle de Bordeaux

[github-actions]

🪟 Windows builds ready!

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(Built from commit dba9913)

[nyalldawson]

For now, it's not possible to render in a CMYK image, and as a consequence to scale it

Out of interest, is addressing this limitation on kdab's roadmap? From a niave pov I can't see why scaling a CMYK image is any different then scaling a rgba one

[troopa81]

Out of interest, is addressing this limitation on kdab's roadmap? From a niave pov I can't see why scaling a CMYK image is any different then scaling a rgba one

Quoting KDAB developer own words

• Then the second step would be to support a minimal set of operations
  that make sense and are relatively easy to add: cropping, mirroring,
  90° rotations, and possibly a few others. Scaling is possibly
  already tricky to do correctly.

I take a look on qimage.cpp transform method (in charge of scaling) and there is two way of scaling

• calling smoothscaled method
• painting with a transform matrix

The second wouldn't work as painting on CMYK image is not possible. Regarding the first, CMYK8888 format is not among the smoothscaled allowed formats, so that's maybe an oversight (I'm gonna reach out KDAB developer regarding this). Funny thing is I can force call of smoothscaled method by asking a 2 times smaller scale and it works without error.

Anyway, long story short, we cannot make assumption of what Qt would return when we scale an image, and there is no way to detect if scaling has worked or not (except for the message QPainter::begin: Cannot paint on an image with the QImage::Format_CMYK8888 format that would pop-up). So I propose to get rid of the QgsMessageLog part. What do you think?