The create_context() and ensure_context() sections should be more clear
on the role of the functions, and their eventual caveats, like being
called multiple times.
If your OpenGL driver supports GLX_INTEL_swap_event that means when
glXSwapBuffers is called it returns immediatly and an XEvent is sent when
the actual swap has finished.
Clutter can use the events that notify swap completion as a means to
throttle rendering in the master clock without blocking the CPU and so it
should help improve the performance of CPU bound applications.
