Added support for subsititing $VAR_NAME entries in shaders to enable writing shaders that work across GLSL versions.

include/osg/State

@@ -823,9 +823,12 @@ class OSG_EXPORT State : public Referenced
           * during rendering. */
         inline void setDisplaySettings(DisplaySettings* vs) { _displaySettings = vs; }
 
-        /** Get the DisplaySettings */
+        /** Get the const DisplaySettings */
         inline const DisplaySettings* getDisplaySettings() const { return _displaySettings.get(); }
 
+        /** Get the const DisplaySettings that is current active DisplaySettings to be used by osg::State, - if DisplaySettings is not directly assigned then fallback to DisplaySettings::instance(). */
+        inline const DisplaySettings* getActiveDisplaySettings() const { return _displaySettings.valid() ? _displaySettings.get() : osg::DisplaySettings::instance().get(); }
+
 
 
         /** Set flag for early termination of the draw traversal.*/