A pointer to the OggVorbis_File structure--this is used for ALL the externally visible vorbisfile
functions.
i
Link to the desired logical bitstream. For nonseekable files, this argument is ignored. To retrieve the vorbis_comment struct for the current bitstream, this parameter should be set to -1.
Return Values
Returns the vorbis_comment struct for the specified bitstream.
NULL if the specified bitstream does not exist or the file has been initialized improperly.