Added support for specifying external overlay file.
Added support for specifying external overlay file.


omega-hub/osg,ghub/osg,ghub/osg,koshman86/osg,koshman86/osg,ntoand/osg,omega-hub/osg,ghub/osg,XiaoCC/osg,artoolkit/osg,stmh/osg,artoolkit/osg,XiaoCC/osg,artoolkit/osg,stmh/osg,XiaoCC/osg,ghub/osg,ntoand/osg,stmh/osg,stmh/osg,koshman86/osg,omega-hub/osg,ntoand/osg,ntoand/osg,koshman86/osg,omega-hub/osg,XiaoCC/osg,XiaoCC/osg,artoolkit/osg,stmh/osg