Pause playback/record when document is hidden (and not presenting)

This will prevent weirdness when the `requestAnimationFrame` is not running because the window/tab is in the background.

- [ ] Do not allow playback or pause when `document.hidden`
- [ ] Pause playback on `"visibilitychange" if `document.hidden`
- [ ] Stop recording on `"visibilitychange" if `document.hidden`
- [ ] Resume playback on `"visibilitychange" if `!document.hidden`