ffmpeg: dont allow -flags to override -pass

Fixes Ticket2154
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -1146,9 +1146,11 @@ static OutputStream *new_video_stream(OptionsContext *o, AVFormatContext *oc, in
         if (do_pass) {
             if (do_pass & 1) {
                 video_enc->flags |= CODEC_FLAG_PASS1;
+                av_dict_set(&ost->opts, "flags", "+pass1", AV_DICT_APPEND);
             }
             if (do_pass & 2) {
                 video_enc->flags |= CODEC_FLAG_PASS2;
+                av_dict_set(&ost->opts, "flags", "+pass2", AV_DICT_APPEND);
             }
         }