BUG #15153: Update color map for the LOD mapper.

We were not setting the ColorTransferFunction on the polydata mapper
used for rendering LOD surface when volume rendering unstructured grids.
Fixed that.

Change-Id: Iccea62555ab9be58af8cbd43544521eb4636bd2c
Merge topic '15153_fix_volume_colormap' into next

71054832 BUG #15153: Update color map for the LOD mapper.