Fix rayrend clipping for clipping plane

* fix upside-down intersection plane
  (clip-normal points outside not inside)
* fix clip-normal in blinnphong-shading
  (do not apply tex2vol() on volume coordinates again)

Note:
  Clipping with spheres still does not work.
Fix local illumination at bounding box

* entering bounding box behaves just like a clipping plane
* intersectBox() also calculates normal of intersected side

Note:
  Due to limited granularity, edges between clipping planes and
  bounding box create and stepped look depending on current quality
  level