Interactively adjust sample distance for vtkSmartVolumeMapper

Added new flag called InteractiveAdjustSampleDistances that
essentially controls whether the internal volume mappers compute
or use a set sample distance. The decision is based on whether
the render is interactive or still depending on the difference
between the DesiredUpdateRate of the vtkRenderWindow and the
InteractiveUpdateRate of the vtkSmartVolumeMapper