unsharpMaskImage in Imagick's thumbnail_image is not compatible with CMYK jpegs.

[gitlost]

This has been split off from #39216.

The use of Imagick::unsharpMaskImage() in WP_Image_Editor_Imagick::thumbnail_image() on jpegs with CMYK color spaces results in mangled images, eg black text becomes fuzzy and over-black, and colors change. The attached patch just doesn't call it if the image has a CMYK color space.

The unit test requires the attached "test_cmyk.jpg" to be uploaded to the test data images directory (which is actually the "full" jpeg produced by WP_Image_Editor_Imagick when processing the "test_cmyk.pdf" PDF uploaded to #39216).

[gitlost]

Don't call unsharpMaskImage() if CMYK color space.

[gitlost]

Test CMYK jpeg. Needs to uploaded to tests data image directory.

[gitlost]

Looks like it's probably this ​ImageMagick bug 299 - after manually updating ImageMagick on Ubuntu 16.10 to 6.9.6-8 (the standard package is at 6.8.9-9) then convert test_cmyk.jpg -unsharp 0.25x0.25+8+0.065 out.jpg works correctly, whereas before it didn't.

[gitlost]

Refresh and fix, with ImageMagick version check.

[gitlost]

On investigating #40537, can confirm that this affects ImageMagick versions 6.8.4-0 to 6.9.6-4 as mentioned in the ImageMagick bug 299 (or at least can confirm it on testing versions 6.8.3-10, 6.8.4-10, 6.9.6-4 and 6.9.6-5 - couldn't find any releases from 6.8.4-0 to 6.8.4-9 to test). So affects the ​Ubuntu ImageMagick packages that come with Vivid 15.04, Wily 15.10, Xenial 16.04 and Yakkety 16.10.

The new patch refreshes and fixes the old patch which was broken anyway, and adds a check to only apply the workaround for these ImageMagick versions.