colour: improve ICC compatibility check for CMYK images

[kleisauke]

See: #3564.

Targets the 8.14 branch.

[jcupitt]

This looks too easy! I can't help feeling there's some corner case we've overlooked, but I don't know what it is.

[kleisauke]

As well as checking it against the test case in #3564, I also checked it against issue #3139 and the test cases in PR #3372. But indeed, perhaps we overlooked something, I will verify some old issues.

[jcupitt]

I checked it against CMYKs with sRGB profiles, RGBs with CMYK profiles, and mono images with RGB profiles, and it all seems fine. I didn't try alpha channels.

I think maybe I'm only uneasy because I remember changing this logic before and breaking everything :( It's probably fine.

[kleisauke]

The test case mentioned in #730 (comment) that was fixed with PR #3139 also still produces the expected behavior.

I'll do some more testing with CMYK images, since that's the only behavior change in this PR (the change in commit bec4942 is also a behavior change, but that's much more uncommon, I think).

[kleisauke]

I've double-checked it with a couple of CMYK images with ICC profiles and didn't encounter any issues. Let's merge.