[vtkusers] memory leak in vtkFixedPointVolumeRayCastMapper

[xabi riobe]

hello,

i have a memory leak in vtkFixedPointVolumeRayCastMapper that can be
corrected adding the following lines at the end of method
UpdateMinMaxVolume:

  delete minNonZeroScalarIndex;
  delete minNonZeroGradientMagnitudeIndex;


Xabi.