Initial email from Tim : "I've implemented using a timestamp, available with ARB_timer_query and OpenGL 3.3, to gather GPU stats. This is nice because it can accurately fix the GPU draw time with respect to the other times on the stats graph, rather than having to estimate the wall time of the end of GPU drawing. This also prevents anomalies like the GPU phase starting before the draw phase..."
Changes to Tim's submission by Robert: Removal of need for swap buffer callback in ViewerBase.cpp, by
integrating a osg::State::frameCompleted() method that does the stats timing collection. Introduction of a
GraphicsContext::swapBuffersCallbackOrImplementation() method that calls the State::frameCompleted() and
the swap buffers callback or the swapImplementation as required.
avoid threading issues associated with compile running in a parallel with
update/cull on the first frame.
Also added automatic recompile when a new SceneData is applied to a View.
