Color protocol

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Abstract superclass for colors:

All color objects are required to implement a method on the >rgba generic word.

Optionally, they can provide methods on the accessors red>>, green>>, blue>> and alpha>>, either by defining slots with the appropriate names, or with methods which calculate the color component values. The accessors should return color components which are real numbers in the range between 0 and 1.

Overriding the accessors is purely an optimization, since the default implementations call >rgba and then extract the appropriate component of the result.