[cairo] Adding a color management interface to GTK

[Bill Spitzak]

I think any attempt to do color management is misguided.

We already have a floating-point api for specifying the colors. This 
means that the sRGB primaries can support an infinite gamut, as I have 
argued earlier.

It is true that images used as sources can only contain clamped sRGB 
values so they have limited gamut. I think it would be far more 
effective to support float and half-float image formats so that an 
unclamped value can be in the image.

Any time spent on "color management" that could have been spent 
supporting floating point image data is time wasted, imho.

If we are going to support any color space control at all, I want to see 
linear sRGB before anything else (this is sRGB primaries but the values 
are directly proportional to the amount of energy emitted, rather than a 
gamma curve). The conversion from linear sRGB to XYZ and to any other 
set of primaries is a 3x3 matrix multiplication and can be done at 
blinding speed by graphics cards. Non-linear such as sRGB are somewhat 
slower but not impossible.