[vtkusers] Color of isosurface

[Dale "Luke" Peterson]

> Ok.  I'll try this, but I'm confused why the same code (and identical
> subclass of vtkImplicitFunction) of my subclassed QVTKWidget would
> display in red and not have this issue.

By this I mean, if I take the same code out of the QVTKWidget and just
run it in main(), and add the neccessary code to create a
renderWindowInteractor(), this surface shows as bright red.