First step of refactoring the osg::Program management in osgTerrain::GeometryPool for better performance by placing default osg::Program and shader defines on a single StateSet below the osgTerrain::Terrain

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14704 16af8721-9629-0410-8352-f15c8da7e697

include/osg/StateSet

@@ -327,7 +327,7 @@ class OSG_EXPORT StateSet : public Object
 
 
         /** Set the list of defines to pass on to shaders.*/
-        void setDefineList(DefineList& dl) { _defineList = dl; }
+        void setDefineList(const DefineList& dl) { _defineList = dl; }
 
         /** Get the list of defines to pass on to shaders.*/
         DefineList& getDefineList() { return _defineList; }