for this image.
Referencing this property has a number of advantages over
referencing the PixMap.ColorSpace.ColorSpaceType property.
Firstly it avoids the possibility that a new ColorSpace object
will need to be created.
Secondly it takes account of the fact that certain types of
PixMap (e.g. ImageMasks) do not have a ColorSpace. In these cases
the ColorSpaceType is implicit in the definition of the PixMap and
needs to be inferred by the PixMap itself.