oggparseopus: export pre-skip

Bug-Id: 945, along with the following commit

libavformat/oggparseopus.c

@@ -57,6 +57,8 @@ static int opus_header(AVFormatContext *avf, int idx)
         st->codecpar->channels   = AV_RL8(packet + 9);
         priv->pre_skip        = AV_RL16(packet + 10);
 
+        st->codecpar->initial_padding = priv->pre_skip;
+
         extradata = av_malloc(os->psize + AV_INPUT_BUFFER_PADDING_SIZE);
         if (!extradata)
             return AVERROR(ENOMEM);