Made the `FPS` effect work properly

  There were two problems, either of which alone were enough to
  erroneously cause the `tick` command to always return
  `True` (signalling that a new frame should be drawn). In Interact.idr,
  the way `requestAnimationFrame` works was causing `yield` to return
  absolute time offsets rather than deltas since the last `yield`. And
  in Main.idr, the handler for `Yield` was computing the next frame time
  as if the parameter represented frames per *millisecond* rather than
  frames per second.