Fixed numerical accuracy problem in Trackball manipulator.

include/osgGA/TrackballManipulator

@@ -49,6 +49,10 @@ class OSGGA_EXPORT TrackballManipulator : public CameraManipulator
         /** For the give mouse movement calculate the movement of the camera.
             Return true is camera has moved and a redraw is required.*/
         bool calcMovement();
+        
+        void computeLocalDataFromCamera();
+        
+        void computeCameraFromLocalData();
 
         void trackball(osg::Vec3& axis,float& angle, float p1x, float p1y, float p2x, float p2y);
         float tb_project_to_sphere(float r, float x, float y);
@@ -68,6 +72,10 @@ class OSGGA_EXPORT TrackballManipulator : public CameraManipulator
         float _minimumZoomScale;
 
         bool _thrown;
+        
+        osg::Vec3   _center;
+        osg::Quat   _rotation;
+        float       _distance;
 
 };