[cairo] Spot colors (and CMYK)

[Bill Spitzak]

I think I am not explaining the API I want clearly.

What would be sent to cairo is "This is the spot color id, and here is 
an sRGB approximation". This sRGB value would be the Pantone CMYK 
converted using the DeviceCMYK to sRGB, although not be clamped to the 
0-1 range (so outside-sRGB-gamut inks can be approximated).

A "smart" driver can ignore the sRGB approximation and do exactly the 
right thing by using the spot color id.

A "dumb" driver will ignore the spot color id and is going to get the 
color wrong. I have yet to see any kind of argument that somehow the 
dumb driver is going to do better if given CMYK and a color profile 
rather than sRGB. That color profile is apparently a constant so it 
should not be needed. The only plausable argument is that there is 
information in the 4-D CMYK space that is lost when converting to 3-D 
sRGB but so many of the color experts here say with absolute certainty 
that XYZ or Lab (both 3-D spaces) will do the job, that it appears this 
is false.