Added 'toColor' and 'fromColor' methods to raster.

According to the javadoc : "a Raster encapsulates a DataBuffer that stores the sample values and a SampleModel that describes how to locate a given sample value in a DataBuffer"

So I think there are two parts in a raster: the part that interprets the raw data of the buffer as a color (toColor and fromColor methods) and a part that give the correct cell of the buffer for the given coordinates  (the coordinateToOffset method).

This separation allows to implement both aspect independently, while the implementation of the read and write methods uses both aspects simultaneously.

This allowed me to implement the removeTransparency method of Image (why does it use java.awt.Color ?)