[cairo] Creating CMYK and spot colors using Postscript backend

[Bill Spitzak]

Craig Ringer wrote:

> IMO it'd be better to simply fail to convert the colour and report to
> the user that the surface only supports RGB colours

I don't think it should ever fail, it should always produce a color, and 
I really don't care how bad that color is.

The reason is that programmers do not want to put "if" statements into 
their rendering code to get it to produce output on multiple devices. If 
they have a fancy illustrator program that can do spot colors, it is 
really nice if exactly the same code can draw a preview on the screen.

If the color conversion is crappy then the programmer can put the if 
statements in. However this is still a much better situation than being 
forced to put them in in the first place.

In general, throwing errors or producing no output makes graphics 
programs extremely fragile and bug-prone and should never be done if at 
all possible. Another example is that poorly-formed UTF8 should draw as 
many glyphs as possible and must always draw a box or something for 
every erroneous byte.