To implement a MOD variant for the Chewy engine, this commit makes some changes
to make subclassing of the ProtrackerStream and associated Module class easier.
A large part of cadavre.mod is broken. No new notes play, and only
the old background sounds remain. It's possible, perhaps even
probable, that the original MOD player didn't have this problem,
but all standalone players I've tried do, so I'm assuming it's the
file that's broken. We work around it by changing the length of
the song after it's been loaded to only include the good parts.
