Backport 'reslicemapper-border' to release branch.

When MultiSamples are used in VTK (which is the default on Windows
and Linux), a thin border appeared at the edge of the image if the
the image pixel values were not zero at the edge.  The thin border
is the result of the zero 'Background' value of vtkImageReslice.
All of the "Background" pixels produced by vtkImageReslice are
supposed to map to beyond edges of the polygon used to render the
image texture.  However, MultiSamples causes the edge to be soft,
and the coloring of these background pixels bleeds into the edge
of the polygon.

The solution is to tell vtkImageReslice to add a bit more edge
tolerance when clamping its interpolation coordinates.  This moves
the 'Background' beyond the edge of the polygon, so that these
background pixels no longer bleed into view.