Fix fullscreen iframe zoom bug.

To reproduce:
- On chrome, zoom browser on a mediaelement in an iframe.
- Go in fullscreen
- Full screen will be exited immediately

The fix:
- Use window.devicePixelRatio for zoom ratio.
- When fullscreen, calculate window width with devicePixelRatio taken into account
Merge pull request #1070 from lisbakke/master

Fix fullscreen iframe zoom bug.