Swap buffers after finishing a compositing cycle
The compositing timing algorithm assumes that glXSwapBuffers() and eglSwapBuffers() block. While this was true long time ago with NVIDIA drivers, nowadays, it's not the case. The NVIDIA driver queues several buffers in advance and if the application runs out of them, it will block. With Mesa driver, swapping buffer was never blocking. This change makes the render backends swap buffers right after ending a compositing cycle. This may potentially block, but it shouldn't be an issue with modern drivers. In case it gets proven, we can move glXSwapBuffers() and eglSwapBuffers() in a separate thread. Note that this change breaks the compositing timing algorithm, but it's already sort of broken with Mesa drivers.