BUG: #2999 Snap to center of pixels when loading volumes

As described in bug report, centering the view to the center
of the pixel makes more sense because it avoids rounding errors
that can occur when there are an even number of pixels and the
slice ends up exactly at the boundary between slices.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21976 3bd1e089-480b-0410-8dfb-8563597acbee