Using Mix_HookMusic() to generate the OPL output stream works fine if
you only ever play OPL music and don't want to use the normal music
output functionality of SDL_mixer. However, now that it's possible to
use substitute music packs with any music output type, this is no longer
the case. Using Mix_HookMusic() disables normal output from eg. Ogg and
FLAC playback. As an alternative, use the post-mix hook and mix the
output from the OPL emulator there.
Cleanup fix as part of #440.
3bde136c60