Eagerly schedule the next animation callback at the beginning of the
frame. If the scheduler queue is not empty at the end of the frame, it
will continue flushing inside that callback. If the queue *is* empty,
then it will exit immediately. Posting the callback at the start of the
frame ensures it's fired within the earliest possible frame. If we
waited until the end of the frame to post the callback, we risk the
browser skipping a frame and not firing the callback until the frame
after that.