Added previous signature back for MapScalars() method

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
94498364811db232990cc7eabb5497d6cb55d3a2. Hat tip to Marcus Hanwell
for identifying the problem with the previous commit and suggesting
the fix.

Change-Id: I78c05ae4041bb07c9ec3978c4e6cc6e80433d871
Merge topic 'MapScalarsBackwardsCompatibility' into master

dafe5a0b Added previous signature back for MapScalars() method