Make reboot reset start time

[casey]

This interferes with the way that we're using reboot in this program:

while(true) {
  reboot();
  rotateColor('green', 0.05 * TAU);
  circle();
  scale(0.5 * elapsed() / 20000);
  wrap();
  for (let i = 0; i < 8; i++) {
    render();
  }
  rotateColor('blue', 0.05 * TAU);
  for (let i = 0; i < 8; i++) {
    render();
  }
  await frame();
}

However, the only reason we're using reboot and not clear() is because all of our functions are stateful. Making them idempotent would mean that we don't have to use reboot to reset the state.