Block opacity can be set while still showing the colormap of the data.
Block color must now be set in order to override the mapper's color map.
Only works when Interpolate Scalars Before Mapping is on and
SetColorModeToMapScalars() has been called (if you want the backface
visible).

Change-Id: I159eeeb068f1ac1fbd49eded0d0f43d6b392508d