Added set/getDistance() method

include/osgGA/MatrixManipulator

@@ -100,6 +100,12 @@ public:
         /** Get the FusionDistanceValue. Used by SceneView for setting up stereo convergence.*/
         virtual float getFusionDistanceValue() const { return 1.0f; }
 
+        /** Set the distance parameter (used by TrackballManipulator etc.) */
+        void setDistance(double /*distance*/) {}
+
+        /** Get the distance parameter. */
+        virtual double getDistance() const { return 1.0; }
+
         /** Set the mask to use when set up intersection traversal such as used in manipulators that follow terrain or have collision detection.
           * The intersection traversal mask is useful for controlling what parts of the scene graph should be used for intersection purposes.*/
         void setIntersectTraversalMask(unsigned int mask) { _intersectTraversalMask = mask; }