trivial: Fix the gtk-doc description for CdIcc
trivial: Add better gtk-doc descriptions for the rendering intents
Add a ICC transform object for simple RGB conversions

We don't want to suggest to applications to link to lcms2 and learn a whole new
library just to convert small images and icons.

Larger or more complex applications will want to use lcms2 for the additional
control over the input and output buffer formats.