QImage: allow for scaling CMYK imagesHEADdev

The codepaths for image scaling are a bit convoluted, so some "surgery"
is needed. QImage::scaled() delegates to transformed(), building a
suitable scaling matrix.

transformed() checks if the matrix is a scaling matrix, and then
has several dispatches.

If smooth scaling was requested:

* if the image format is supported by smoothScaled() without needing a
  conversion, delegate to that;

* otherwise, if the transform is "non paintable" or the source image is
  big enough, then again call smoothScale. "non paintable" here means
  that we're scaling more than 2x down, and QPainter wouldn't do a good
  job.

Otherwise, images in color formats (>= RGB32) are converted by applying
the needed transformation on a QPainter and draw the source image with
that transformation.

Otherwise, if the matrix is invertible (a scaling matrix with non-zero
scaling always is, it's a diagonal matrix), then dispatch to
qt_xForm_helper.

--

Amend this reasoning to support CMYK images:

* Make smoothScaled support CMYK without conversions. To do so,
  make qSmoothScaleImage scale CMYK as if it was a ARGB image.

* Make transformed() call smoothScaled() for CMYK images

* In transformed(), consider CMYK as nonpaintable, because we can't
  paint over a CMYK image.

* In the non-smooth codepath, also check that we don't try to paint over
  CMYK, and always go through qt_xForm_helper instead.

Note that we still don't support any other transformation for CMYK.

Add a test, adapting the exiting one for RGB.

Change-Id: Ic72d78923a17fb3963aa22c57265904c716792b0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

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