fix(videojs-dash): currentTime with DVR live stream

[amtins]

Description

This PR fixes the currentTime function to keep consistency with Http-Streaming. The currentTime function when used with live streams has two problems:

• As a getter, the returned position is expressed in timestamp instead of seconds
• As a setter, the value taken as a parameter must be expressed in timestamp instead of seconds

Screencast.from.17.05.23.16.20.40.webm

Use Case

A developer wishing to redefine the position will have to use a timestamp instead of a position expressed in seconds.

To reproduce the problem open videojs-contrib-dash and in the developer console copy-paste the code below:

player.options({ liveui: true });

player.loadMedia(
  {
    src: { src: 'https://livesim.dashif.org/livesim/testpic_2s/Manifest.mpd' },
  },
  () => {
    player.one('play', () => {
      setTimeout(() => {
        // should seek to half of the live window
        player.currentTime(player.liveTracker.liveWindow() / 2);
      }, 2_000);
    });
  }
);

Specific Changes proposed

• Override tech_ currentTime, setCurrentTime and seekable functions to support lives
• currentTime, returns the position expressed in seconds
• setCurrentTime, takes as a parameter a position expressed in seconds
• seekable, returns a range expressed in seconds

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
• Reviewed by Two Core Contributors