core/iccprofile: add support for the MHC2 tag
It's a non-standard tag Windows supports, and some Windows laptop manufacturers use it in their profiles. While the usage of the calibration matrix seems to be exclusively bad hacks, supporting it properly is better than the profiles just not working at all.
This also opens the door to using the same tag for min/max luminance of HDR displays, but I'll need to look into how to expose that in KScreen first.