BUG: Fixed distorted sphere brush in editor paint tool

Out-of-plane dimension of the sphere brush was computed incorrectly. When a volume with anisotropic spacing was loaded then the bursh shape became ellipsoid.
(e.g., load the MRHead volume, set Z spacing to 10mm, draw in the sagittal view => enormous blobs appear on the axial and coronal slices).

The problem was that out-of-plane voxel size was computed incorrectly.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23464 3bd1e089-480b-0410-8dfb-8563597acbee