Return the elapsed time in milliseconds since the previous call of this
method by subtracting the previously given currentTime argument from the
current currentTime argument.
Takes visibilitychange events into account.
Current time acquired e.g. with requestAnimationFrame.
A helper class for calculating the elapsed time between frames.
Handles the case where the browser tab is not in focus and
requestAnimationFramedoes not get called for performance reasons. In this case, the returned deltaTime won't be an unreasonably large value.