avformat/movenc: auto insert vp9_superframe bsf when needed

Experimental VP9 support was added to the muxer recently.
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/movenc.c

@@ -6352,6 +6352,8 @@ static int mov_check_bitstream(struct AVFormatContext *s, const AVPacket *pkt)
     if (st->codecpar->codec_id == AV_CODEC_ID_AAC) {
         if (pkt->size > 2 && (AV_RB16(pkt->data) & 0xfff0) == 0xfff0)
             ret = ff_stream_add_bitstream_filter(st, "aac_adtstoasc", NULL);
+    } else if (st->codecpar->codec_id == AV_CODEC_ID_VP9) {
+        ret = ff_stream_add_bitstream_filter(st, "vp9_superframe", NULL);
     }
 
     return ret;