Clean up `epilogue` track definition by allowing subtracks to loop

[CPColin]

Right now, the epilogue track uses a source that's too short for the whole Zelda epilogue and has to use subtracks to play it twice, back-to-back, so it can last for the whole cutscene before fading out. It could make more sense if subtracks could loop. We don't want all of them to loop, though, or that would screw up tracks that add one-time sound effects. (It would make it easier to add an AWOOGA to the Escape theme, though.)

See if this makes sense to implement.