The actual data was updated correctly in the BoxWidget
code, but the wrong object was being marked as modified.

The rendering code got smarter at some point and now
only rebuilds VBOs when the corresponding cell array
has been modified. Marking the entire polydata as modified
is no longer sufficient to update the on-device
representation of the highlighted face.

This patch fixes the BoxWidget so that the active face is
used for highlighting instead of getting "stuck" on an
arbitrary face.