Fade out music at end of level, before intermission

Small polish feature to fade out the level music before intermission, like Sonic 3.

Test cases, please break:

• Single player
• Splitscreen: Co-op with playersforexit One (one player needed to exit the level)
• Splitscreen: Co-op with playersforexit All (all players needed to exit the level)
• Splitscreen: Race when all players finish (exits immediately, no fade)
• Splitscreen: Race when player finishes during the countdown (exits immediately, no fade)
• Splitscreen: Race when player does not reach the end (countdown music finishes, does not fade)
• Splitscreen: Ringslinger (does not fade at end of level; should we support this? I'm thinking no.)
• Netgame, for all the above
• Headless server (render_mode is none)
• Anything else?

musicplus-endoflevel