BUG: The vtkCellPicker was not returning image mappers.

When vtkImageActor got a mapper (in July 2011), I apparently forgot to
modify vtkCellPicker so that it would return that mapper.  Better late
than never.

Change-Id: I5a3bca3d610c5dab34db4a70430e8ca575297633
Merge topic 'picker-image-mapper' into master

b327246c BUG: The vtkCellPicker was not returning image mappers.