Created a new GraphicsThread subclass from OperationThread which allows the
GraphicsContext specific calls to be moved out of the base OperationThread class.
Updated the rest of the OSG to respect these changes.
as osg::GraphicsOperation. Unpdated parts of OSG depending upon these.
Added a virtaul bool valid() method to osg::GraphicsContext to allow apps to
test whether a valid graphis context has been created or not.
added frame stamp updating and update traversal to osgViewer::Scene/Viewer.
Updated osgcamera example to use new Viewer API calls instead of using local
rendering calls.
the names of the operations to be logged for stats purposes, or used when
do searches of the operation list. The keep member variable tells the graphics
thread run loop wether to remove the entry from the list once its been called.
