BUG: Fixed opacity mapping when scalar array is vtkUnsignedChar.

Incorrect logic in vtkDiscretizableColorTransferFunction::MapScalars()
ended up interpretting the presence of vtkUnsignedCharArray for scalar
as meaning that the transfer function is not mapping scalars to colors
but directly rendering them. This lead
vtkDiscretizableColorTransferFunction is not do opacity mapping even
when requested by user. Fixed the check and added some documentation for
that check to better explain its purpose.

Change-Id: Ic8f4c0188194a363a8640befe1bd4090acc5c0ff
Merge topic 'fix_opacity_mapping' into master

a5daf047 BUG: Fixed opacity mapping when scalar array is vtkUnsignedChar.