Display cover image in jukebox mode (the layout could be better but this is a good starting point).
Implement better fallback for songs with no background image or video.

- Tiled screen appears only when the playing song is changed
- Heuristics for determining if the background/cover should be tiled or fullscreen
- Background is not drawn behind a video
- Tiling drawn as one Surface::draw call
- Animation to keep things more lively
- Added pixel width/height accessors to Surface