* configure.ac: detect if libavformat is present.
* modules/codec/ffmpeg/demux.c: new experimental demuxer using libavformat (most of the work done by fenrir).
   This demuxer should allow us to support "frindge" formats (FLIC, SWF, etc...).
   The demuxer has been put here because of its dependancy on libavformat and libavcodec.
* modules/codec/ffmpeg/ffmpeg.c: re-arranged the codecs list.