Added default setting of _threading_model to ThreadPerCamera when multiple cameras are present, and SingleThreaded when a single camera is being used.
Added default setting of _threading_model to ThreadPerCamera when multiple cameras
are present, and SingleThreaded when a single camera is being used.


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