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


artoolkit/osg,artoolkit/osg,jtorresfabra/osg,jtorresfabra/osg,tomasthoresen/osg,scrawl/osg,marchelbling/osg,openscenegraph/osg,openscenegraph/osg,scrawl/osg,marchelbling/osg,jtorresfabra/osg,marchelbling/osg,marchelbling/osg,tomasthoresen/osg,jtorresfabra/osg,artoolkit/osg,scrawl/osg,openscenegraph/osg,openscenegraph/osg,artoolkit/osg,tomasthoresen/osg,scrawl/osg,tomasthoresen/osg