metadata: get embedded cover art from video files using libavformat

Newer version of libavformat can grab cover art, so we'll go ahead
and use it.  We also need to make sure we don't confuse cover art
with a normal video stream.

libav.h

@@ -163,3 +163,20 @@ lav_get_interlaced(AVCodecContext *vc, AVStream *s)
 	return (vc->time_base.den ? (s->avg_frame_rate.num / vc->time_base.den) : 0);
 #endif
 }
+
+static inline int
+lav_is_thumbnail_stream(AVStream *s, uint8_t **data, int *size)
+{
+#if LIBAVFORMAT_VERSION_INT >= ((54<<16)+(6<<8))
+	if (s->disposition & AV_DISPOSITION_ATTACHED_PIC &&
+	    s->codec->codec_id == AV_CODEC_ID_MJPEG)
+	{
+		if (data)
+			*data = s->attached_pic.data;
+		if (size)
+			*size = s->attached_pic.size;
+		return 1;
+	}
+#endif
+	return 0;
+}