Fixed bug causing plane to disappear on boundary.

When the plane was moved to one of the sides of the bounding region, and
was parallel to the bounding region, it sometimes disappeared. To fix this
the origin of the plane is clamped to slightly within the bounding region.

Change-Id: Ie4c22df9faf9d6dc739dd542f07df136f0784e9d
Merge topic '0014066-vtkImplicitPlaneWidget2-disappears' into master

91045594 Fixed bug causing plane to disappear on boundary.