Flushes any requests queued for the windowing system.
This happens automatically when the main loop blocks waiting for new events, but if your application is drawing without returning control to the main loop, you may need to call this function explicitly. A common case where this function needs to be called is when an application is executing drawing commands from a thread other than the thread where the main loop is running.
This is most useful for X11. On windowing systems where requests are handled synchronously, this function will do nothing.