BUG: Fixed slice spacing computation

Slice spacing was incorrectly computed for non-axis-aligned volumes.

This caused issues such as being able to go outside a single-slice volume in a slice view when with the arrow key. What the user saw was that when he accidentally moved the mouse wheel then the image disappeared from the slice viewer.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25694 3bd1e089-480b-0410-8dfb-8563597acbee