Fixes for composite poly data mapper2

Block Opacity was not being taken into account when calling
MapScalars in vtkMapper. Now does a better job copying values
from the mapper to the helpers. Minor cleanup on replace.

Change-Id: I01f708441a0021d9261e265466250826af9a9566
Merge topic 'composite_fix' into master

5cc2de81 Fixes for composite poly data mapper2