Added osg::SyncSwapBuffersCallback to include/osg/GraphicsContext and support for enabling it to include/osg/DisplaySettings, and to the Viewer/CompositeViewer::realize() methods.

To enable the sync of swap buffers set the env var OSG_SYNC_SWAP_BUFFERS to ON or 1, to switch off set to OFF or 0.

One can also use the --sync command line option for application that pass on command line options to the osg::DisplaySettings::instance().


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14456 16af8721-9629-0410-8352-f15c8da7e697
Removed --sync related code as it's now integrated into osg::DisplaySettings and osgViewer


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14457 16af8721-9629-0410-8352-f15c8da7e697