fftools/ffmpeg_demux: do not set bitexact directly on the decoder

Add it to decoder options instead, to be processed when opening the
decoder. This way it won't be overridden by flags the user might be
setting otherwise.

fftools/ffmpeg_demux.c

@@ -1156,7 +1156,7 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
     }
 
     if (o->bitexact)
-        ist->dec_ctx->flags |= AV_CODEC_FLAG_BITEXACT;
+        av_dict_set(&ist->decoder_opts, "flags", "+bitexact", AV_DICT_MULTIKEY);
 
     switch (par->codec_type) {
     case AVMEDIA_TYPE_VIDEO: