Poster never displays if not set on creation

[dmlap]

If no poster image is assigned when video.js is initialized, the poster div is explicitly hidden. If a poster is assigned later, the poster component is never re-shown.

[heff]

Everything is set up to make that work except we need to switch the poster css to use vjs-has-started (to hide it) and remove the on('play', hide)

[heff]

Found one odd browser behavior here that's worth noting. In Chrome/Safari, if you set the video element's poster after the video has started playing, the poster will show and hide the video while the video keeps playing. I can't think of a real use case for doing that, but Firefox doesn't show the poster, which seems more correct.

[heff]

closed by #1568