Fade out music when dying and switching levels

A small polish feature, so the music fades out when you die like the classic games.

This accomplishes the following:

1. Fade out old music when switching levels
2. Fade out old music when dying AND resetmusic on (OR always when the new music is different)
3. Set resetmusic on by default
4. Add a menu toggle for resetmusic

Please break this; I would appreciate it. Test cases:

• Single player: Dying with lives
• Single player: Dying and game over (level music should not fade, but continue music should fade out before new map)
• Splitscreen co-op and race (nothing changes, since the music doesn't reset unless you switch levels)
• Splitscreen ringslinger (nothing changes, since the music doesn't reset unless you switch levels)
• Headless server (render_mode is none)
• Netgame anything???

musicplus-dying-demo