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/TextBase

@@ -337,6 +337,7 @@ protected:
     osg::Vec3                               _offset;
     osg::Vec3                               _normal;
     osg::BoundingBox                        _textBB;
+    osg::BoundingBox                        _textBBWithMargin;
 
     mutable osg::Matrix                     _matrix;