workshop: add a gray component replacement implementation
This implementation assumes that C=M=Y is gray, this could be improved by extracting some more information from the ICC profile of the space. However the changes when converted to RGB for display by GEGL using applications will be wysiwyg. For now the inklimit slider has to be pushed to the right for the final render. It might well be better to have two separate ops for this and juggle them in ui code instead.
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