[vtkusers] coordinate transformation between polydata and image data

[David Doria]

imageData->FindPoint(x,y,z) will give you the index (vtkTypeId) of the
pixel/point in the ImageData that (x,y,z) is closest to.

I wrote a patch a while ago to convert from this linear index back to
(i,j,k) image coordinates, but I forgot to add reviewers!
http://review.source.kitware.com/#change,466 Hopefully you can look
for this soon.

David