Clean up music player

[lauwaichung]

Reason

This stuff is all very non-essential. I was just playing around with the game, wanted to add my own music in, and I noticed a couple opportunities to clean up.

Changes

• fix shuffle
  • was shuffling a list of indices, but then ignored that list
  • now just shuffles array of tracks
• remove null tracks
• move update code to coroutine
  • not necessary to finish changing tracks during a single frame
• destroy if no tracks