Added options into osgprerender for controlling how to do the pre rendering i.e.
--fbo, --pbuffer, --fb --window, and also added the option for controlling the
window size with --width and --height.
Producer xplatform support for pbuffer instead of Win32 specific pathways
as support before. (with a few code tweaks to simplfy the code, by Robert Osfield).
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.
