Most of the synchronization was too deep in the implementation so that it did not guard everything it needed.
Now each song has precisely one mutex which must be locked for all access to its internals - this is done in the public ZMusic interface
Now there is only one single entry point for both, instead of previously 2 entry and 4 exit points.
This also eliminates the explicit shutdown of ZMusic. Timidity++'s two buffers have been put in containers that self-destruct on shutdown and calling dumb_exit is not necessary because the only feature requiring it is not used by any code in the music library.
Also made the decode_vorbis function in DUMB a function pointer so that the library does not depend on high level code and can just ignore the vorbis case if no supported.
