Implement two SDL backend modes.

  SDL with mutexes;
    - it waits if buffer is full until it has some free space.
      More accurate, but fast-forward is not possible.

  SDL with SDL_LockAudio/SDL_UnlockAudio
    - it discards sample data which don't fit into buffer.
      Less accurate, but fast-forward is possible.

  Use --sound-sdl for mutexes and --sound-sdl-old for the second algo.