demux_lavf: add a hack to work around other hacks

mplayer (and all forks) use a single FourCC field to map codecs. This is
what sh_video->format and sh_audio->format is used for. Basically it's a
key into the codecs.conf table to find out what decoder to use. (There's
only one video codec and one major audio codec left - both libavcodec.
But we still have to deal with mapping coming from non-libavformat
demuxers.)

It seems this causes some video codecs to fail, such as WV1F in AVI [1].
demux_lavf generally throws away the codec_tag from libavformat if the
codec is MPEG-in-AVI. There are probably other cases like this. Add a
hack to enforce passing the correct codec tag when only ffmpeg demuxers
and decoders involved.

Note: the sample [1] needs to be flipped. With --demuxer=avi, this is
done correctly, because unlike demux_lavf, the demuxer uses the FourCC
directly for the sh_video->format tag, and finds the correct codecs.conf
entry (which contains the flip flag). We could just add a "whitelist" of
codec tags which can be passed through to sh_video->format, but I don't
want to do that just for such an obscure format as the sample at hand.

Note 2: when demux_lavf is used, the AVCodecContext could be passed
directly to vd_ffmpeg/ad_ffmpeg. The code to convert to/from the
internal mplayer stream headers is probably still needed, as there are
non-ffmpeg demuxers and audio decoders.

[1] http://samples.mplayerhq.hu/V-codecs/WV1F/AVI/title2.avi

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