vtkVolumeRayCastSpaceLeapingImageFilter is a multi-threaded optimized
imaging filter that computes the datastructures needed to skip empty
regions in the scalar opacity or the gradient opacity transfer functions.

This has several improvements over the earlier version.
1. Its multi-threaded now
2. The computation of the scalar opacity and gradient opacity empty
   regions are visited in a single pass through the data as opposed
   to two separate passes, earlier
3. Lots of loop unrolling / precomutations and optimizations
4. The block size for space leaping can be changed by changing the
   ifdef in the C++ file. Its not an ivar for possible slowdowns in
   inlining.