leaving it up to the Viewer to specify the mode (which by default is MultiThreaded).
Added a check for the presence of osgParticle systems so that threading is
disabled in this case.
osgSim::LightPointNode, the max pixel size from was originally in osg::LightPoint.
Added additional get/set methods to osg::LightPointNode.
Increased the compute pixel size by a ratio 1.41 (sqrtf(2)) to correlate the
computed size with the actual size seen on screen.
