Fix clipping modes for clipping sphere

* use one parameter _clipMode for distinct clipping mode
  (0=off,1=plane,2=sphere)
* do not share data within independent clipping modes
* allow vview to iterate between clipping modes
* disable plane perimeter for non-plane clipping modes

Note:
  Clipping still does not work within all renderes.
  Clipping sphere can be enabled with vview for rayrendering by
  pressing 'I'-key twice, but sphere position and size are not
  editable yet.