Even though the MapScalars() method is designated an internal method
in vtkScalarsToColors and vtkDiscretizableColorTransferFunction
classes, backwards compatibily for outside classes that override this
method was broken by a signature change for the method in commit
94498364. Hat tip to Marcus Hanwell
for identifying the problem with the previous commit and suggesting
the fix.

Change-Id: I78c05ae4041bb07c9ec3978c4e6cc6e80433d871