* I don't like the indentation style at all, but for the most part
  I've left it alone.

* Much of the code looks ugly to me.

* The return value from SDL_RWread() is checked inconsistenly.

* Group the members of MidiSong into logical units, i.e. structs?

* The debug messages are probably a bit too noisy. I've removed one
  particularly annoying one, but...

  Some of them should be turned into error messages instead.

* Can the instrument handling be made more efficient? At the moment
  different MidiSongs may separately load the same instrument.

  Note that the MidiSong's audio format affects how the instrument is
  loaded, so it's not as easy as just letting all MidiSongs share tone
  and drum banks.

  At the moment they do share the data that is simply read from the
  config file, but that's just a quick hack to avoid having to read
  the config file every time a MIDI song is loaded.

* Check if any of MidiStruct's members can safely be made into static
  globals again.

* TiMidity++ adds a number of undocumented (?) extensions to the
  configuration syntax. These are not implemented here. In particular,
  the "map" keyword used by the "eawpats".

* The other decoders generally only read as much of the file as is
  necessary. Could we do that in this decoder as well? (Currently it
  seems to convert the entire file into MIDI events first.)

* Can it be optimized?