Fullscreen of a player, with the fallback that an iPhone needs, where
only a video element itself can go fullscreen, in the system's own
player.

The functions take these opts:

- :video, the video element of the player, for that fallback

    (-> (enter! player {:video video :orientation :landscape})
        (.then #(swap! page assoc :fullscreen %)))
    ;; {:fullscreen :element :orientation {:refused :not-supported}}
    ;; on a desktop, or {:fullscreen :video …} on an iPhone

(-> (enter! video)
    (.then prn))
;; prints {:picture-in-picture true :width 320 :height 180}, or e.g.
;; {:refused :not-allowed}

Picture-in-Picture of a video: whether it's available, entering and
leaving it, and the size of its window, as data.

Each engine refuses a video in its own words, e.g. a video with
disablePictureInPicture throws an InvalidStateError in Chrome and
Firefox, but a NotSupportedError in WebKit. So enter! checks the video
first, and answers with one keyword for each cause:

    (-> (enter! video)
        (.then prn))
    ;; prints {:picture-in-picture true :width 320 :height 180}, or e.g.
    ;; {:refused :not-allowed}

To keep track of whether the video is in Picture-in-Picture, use
watch!, which also sees the user close the window.

Casting and AirPlay of a media element, from Remote Playback: whether a
device is available, the browser's picker of devices, and the connection
to one, as data.

Firefox has no Remote Playback, so there no device is available and
prompt! answers {:refused :not-supported}. WebKit also fires its older
AirPlay events, which watch! follows as well, since Video.js finds the
standard events unreliable in Safari.

(def stop!
  (hold-while-playing! audio))

The screen kept on while a media element plays, through Screen Wake
Lock.

A playing video keeps the screen on by itself, but no engine does that
for audio, so a page that shows a transcript or lyrics while an episode
plays goes dark:

    (def stop!
      (hold-while-playing! audio))

The functions take these opts:

- :window, the window whose navigator holds the lock, instead of the
  page's, e.g. a stand-in in a test