ffmpeg: init input streams before opening encoders

Codec options of streams detected during avformat_find_stream_info are not set
therefore without this patch we initialize the encoders with decoder info
based on decoders without options.

This cause problems for probed DVB teletext streams where
avctx->subtitle_header depend on the txt_format setting.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Marton Balint <cus@passwd.hu>
