Offset plane bounds
Offset plane bounds

Offset the plane bounds so that the plane does not get clipped and become
unsuable when it reaches the bounds of the data source. This fixes a bug that
would make the plane impossible to select by mouse if it was moved to the maximum
position in one of the orthogonal directions.

Signed-off-by: Brianna Major <6a62c0472e857db5994824f147beeb095c78d3a5@kitware.com>

OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz