A draft fix for CMYK blending modes
This patch is a draft only. We need to decide what to do with the following modes as well (they are not touches atm):
- Over
- Copy
- Alpha Darken
- Erase
- Behind
- Destination In
- Destination Atop
- Greater
- Dissolve
- LuminositySAI
Most probably, all the blendmodes except Greater, Dissolve and LuminositySAI don't need any changes, though it needs checking.