livesim: Automatically tune nbextrastep

[HadrienG2]

Manual selection of the number of simulation steps per frame does not mix very well with frame time budgets and VSync.

Instead, go for the following logic :

• Start at steps_per_image = 1
• Monitor the frame time in the event loop until we have enough statistics to tell if a frame takes abnormally long (need some sort of 95% upper bound). Ignore the first Nswapchain frames, which will have different perf characteristics.
• If framerate is already terrible (<45Hz), don't try to increase steps_per_image.
• Otherwise, double steps_per_image until such an abnormal frame (>95% upper bound) is observed.
• Subsequently decrease steps_per_image by 1, down to a minimum of 1, whenever an abnormally long frame time is observed. With this, steps_per_image will slowly converge to a lower bound of the optimal value, which is better than the visual jank we would observe if we allowed it to go up again (causing steps_per_image to oscillate).
• Log steps_per_image changes to make them observable.
• Use this logic when no nbextrastep is specified on CLI. Otherwise, do as the CLI said.