[cairo] Spot colors (and CMYK)

[ecir hana]

Hello,
just a quick one: what do you think should happen when you have one
surface in duotone (or CMYK or any other non-3D space, as Bill Spitzak
calls them) and you want to paint it over (copy, multiply) 3D color
space? In other words, what do you think, who should do the conversion
- Cairo or the application? But perhaps you, or someone else, see
another possibility?