BUG: fix calculation for snapping to center of slice (fixes 0002878)

When performing pixel level operations, as with the Editor module,
or even when rendering multiple layers, the result can be ambiguous
when the slice plane is exactly at the border between two pixels due
to rounding differences accumulated through transforms. This can lead
to inconsistent rendering or operations being applied in a differnt
location than expected.

A regression from 4.1 had crept in by which the slice plane was
being consistently adjusted to be exactly on the pixel boundary,
leading to the potential issues. This fix ensures that the plane
is adjusted to the middle of the pixel.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21640 3bd1e089-480b-0410-8dfb-8563597acbee