Added conversions to SHW reader and writer that convert the depth to a standard Z ( distance from image plane ), instead of the weird 3delight "spherical distance from the near clip".

This will change the results we get from reading and converting SHWs, but it should be much more consistent now, and will fix the misalignment we've been seeing in Houdini.
Revert this commit for Cortex 9!
Switched from using member variable to using ClassData, in order to preserve binary compatibility.
Merge pull request #287 from danieldresser/shwPerpendicular

Added conversions to SHW reader and writer