The number of color components for each pixel.
For example Grayscale images contain one component, RGB images
contain three and CMYK images contain four.
Referencing this property has a number of advantages over
referencing the PixMap.ColorSpace.Components
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 number of components is implicit in the definition of the
PixMap and needs to be inferred by the PixMap itself.