vtkDiscretizableColorTransferFunction used control points to map indexed colors.
To make applications simpler, added a new API to add indexed colors which are
used when present when in IndexedLookup mode. If no IndexedColors are specified,
the vtkDiscretizableColorTransferFunction reverts to the previous behaviour for
backwards compatibility.

Change-Id: If5b6791baccf2ee0022fd1b1098093d226ef64c3