avcodec: add side_data type for updated metadata

This type is intended to be used to allow codecs to pass updated
metadata to applications.

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>
Reviewed-by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -1044,6 +1044,13 @@ enum AVPacketSideDataType {
      * follow the timestamp specifier of a WebVTT cue.
      */
     AV_PKT_DATA_WEBVTT_SETTINGS,
+
+    /**
+     * A list of zero terminated key/value strings. There is no end marker for
+     * the list, so it is required to rely on the side data size to stop. This
+     * side data includes updated metadata which appeared in the stream.
+     */
+    AV_PKT_DATA_METADATA_UPDATE,
 };
 
 /**