ffmpeg: use a AVDictionary instead of the context to move swr parameters around

This avoids per parameter changes in ffmpeg at the cost of making access
somewhat more annoying.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -3212,6 +3212,7 @@ static int transcode(void)
                 av_freep(&ost->st->codec->subtitle_header);
                 av_free(ost->forced_kf_pts);
                 av_dict_free(&ost->opts);
+                av_dict_free(&ost->swr_opts);
                 av_dict_free(&ost->resample_opts);
             }
         }