[cairo] Color space API

[Bill Spitzak]

Andrea Canciani wrote:

>> I don't currently have a proposal for this since looking at the ICC
>> specification I could not see how to extract this linear space from an
>> ICC profile.
> 
> In general it will not be possible to extract such a color space,
> because ICC profiles can define a color mapping which is almost
> completely arbitrary.

At the middle point where the data is mulitiplied by a square matrix of 
the same size as the number of channels, the data can be assumed to be 
linear.

In any case I recommend that "linear compositing" be done in a fixed 
method that only works "correctly" for sRGB (it will also work for any 
other primaries as long as the gamma is the same curve as sRGB, and will 
be "better" than direct compositing for any colorspace where the gamma 
is greater than 1.5).