When pwf range is much smaller than array range, vtk crashes.

In ParaView, we have to supply the ui with pwf instances for the
opacity and size mapping.  The default pwf range is [0,0] to [1,1].
If you choose to size by an array with a range much larger than this,
ParaView crashes.  You can get around it by first open the pwf
editor and adjusting the x range to match an array you would like
to work with.

This fix could cause the mapper to be too slow, so I would like to
solicit other, more efficient approaches.