Clean up YCbCr colorspace support

A while back, I managed to find the old LCMS test profiles on archive.org, which contained YCbCr profiles (attributed to sun microsystems). I found that there was a very typical assert blocking their use, which I have now fixed.

Things I've tested:

• Blending modes work fine.
• Color smudge has the same problem as for CMYK a while back.
• I have gotten an assert in the histogram. (fixed)

I'll be poking at it for a while, to see what else I can clean up...

Test Plan

Load a YCbCr profile... I am unsure if should be uploading the old test profiles, so just ping me and I'll email them.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

/cc @lsegovia