Refactored the way that margin is applied to the text buounding box to prevent growth of the bounding box each time Text::setPosition() is called.

include/osgText/Text

@@ -282,9 +282,6 @@ protected:
 
     virtual void computePositionsImplementation();
 
-    void computeBackdropBoundingBox();
-    void computeBoundingBoxMargin();
-
     void computeColorGradients();
     void computeColorGradientsOverall();
     void computeColorGradientsPerCharacter();