* modules/demux/ogg.c, modules/demux/mkv.cpp:
  - store the vorbis/theora/speex headers in fmt.p_extra.
  - this change simplifies the code quite a bit and is a lot cleaner.
* modules/codec/vorbis.c, theora.c, speex.c, flac.c:
  - get the headers from fmt_in.p_extra or from the bitstream if empty.
* modules/mux/ogg.c:
  - recontstruct the headers from fmt.p_extra.