media: add web video playback gluelayer

Media playback is delegated to JavaScript. The engine treats your player as a
frame source and drives it through this interface.

```js
Module.createMediaPlayer = (loader, bytes) => new MyPlayer(bytes);

class MyPlayer {
  sync()      // called once per render (state sync)
  play()
  pause()
  stop()
  seek(sec)
  loop(on)
  volume(v)   // 0 - 1
  mute(on)
  dispose()
}

sync() {
  if (!ready) return null;                                                    // not ready
  if (!newFrame) return { width, height, duration };    // sync metadata
  return { width, height, duration, data, time };          // frame delivered
}
```