fix(ImageMapper): Ignore piecewise function for single components
fix(ImageMapper): Ignore piecewise function for single components

Fixes #310, which is caused by discontinuities the user can build
into the piecewise function.  Respecting the transfer function for
single components was added to support images with only a label
map, where we would like to highlight certain labels.  Support for
that will be added back using another approach.

Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js,Kitware/vtk-js