For some codecs, we need to invoke a codec parser (because libavcodec
will run into trouble otherwise). This was done based on the Matroska
codec field.
But this ignores handling of vfw-muxed files, which use a pseudo-codec
to signal presence of vfw structures, which we must unmangle to get the
real codec. Handle this by rearranging the code.
This fixes at least mp3-in-mkv for vfw-muxed files; typically old files.