[cairo] Spot colors (and CMYK)

[ecir hana]

Just to check if I understand this correctly:
- to support CMYK one has to be able to create CMYK solid source
pattern, i.e. _cairo_color struct needs to have other components
besides red, green, blue and alpha?
- to support spot colors... variable number of attributes in
_cairo_color? I'm lost, I don't know... Spot color is just a name
("Ultramarine") which has a fallback in the case the printing device
does not have that particular colorant (because it still might be
possible to print it in subtractive color space) and maybe even one
more fallback if it is displayed on the screen (RGB)? Then, there is
another issue because sometimes it is useful to define multiple spot
colors (i.e. DeviceN) so one could create solid pattern out of
"Ultramarine" and "Silver". (Btw, then CMYK becomes just a special
case of color space with colorants "Cyan", "Magenta", ....)