Fix SpecialColors for vtkWindowLevelLookupTable

When the AboveRangeColor and BelowRangeColor were added to the vtkLookupTable several years ago, the vtkWindowLevelLookupTable was not similarly modified. As a result, the values of the beyond-range colors were left uninitialized and generally had a value of (0,0,0,0) when the vtkWindowLevelLookupTable was used.

I also took the opportunity to have vtkWindowLevelLookupTable implement the ForceBuild() method instead of the Build() method. This allows ForceBuild() and Build() to work for this class in the same way as they do for vtkLookupTable.