Solved bug 363 (still to be improved, though)

Solved bug 363 (still to be improved, though)

- indeed a z-buffer issue (order of painting)
- actors' positions were being chosen wrt to pixel coordinates, instead of world coordinates
- image actors apparently don't can be directly overlaid on top of each other (no positioning issue)
- VF actor is placed right before or after the start of the image actor's bounding box, depending on the position of the camera
- Clipping box of landmarks has also been corrected
- This solution seems to work robustly, but we never know... ;-)
- Will improve VF by using a vtkSlicer object to interpolate between slices