Poster shows through full screen

[joeyparrish]

To reproduce:

1. Run the demo.
2. Select an asset that is a different aspect ratio than your screen. Sintel works for me, but Angel One has a different ratio if Sintel doesn't work for you.
3. Click load.
4. Click the fullscreen button.

The Shaka player poster shows through where there should be black bars.

This slipped in unnoticed in a060bf2 when we went from using poster= in the HTML to using background-image in CSS.

[joeyparrish]

Cherry-picked to v2.1.1.