ffmpeg: fix transcoding dvbsub to dvbsub

fix ticket #2024
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2281,6 +2281,8 @@ static int init_input_stream(int ist_index, char *error, int error_len)
         ist->dec_ctx->thread_safe_callbacks = 1;
 
         av_opt_set_int(ist->dec_ctx, "refcounted_frames", 1, 0);
+        if(ist->dec_ctx->codec_id == AV_CODEC_ID_DVB_SUBTITLE)
+            av_dict_set(&ist->decoder_opts, "compute_edt", "1", 0);
 
         if (!av_dict_get(ist->decoder_opts, "threads", NULL, 0))
             av_dict_set(&ist->decoder_opts, "threads", "auto", 0);