Fix for impostors bug where image insert were visible, this was down to a double

call to RenderStage::drawPreRenderStage().  The call to it in RenderStage inside
the draw() method has been removed, leaving on the call in SceneView.

include/osg/ImpostorSprite

@@ -143,6 +143,8 @@ class SG_EXPORT ImpostorSprite : public Drawable
         /** accept a PrimtiveFunctor and call its methods to tell it about the interal primtives that this Drawable has.*/
         virtual void accept(PrimitiveFunctor& pf) const;
 
+        // for debugging purposes.
+        Vec4 _color;
 
     protected: