Adding support for parsing BlockAdditional

Matroska specification lists support for BlockAdditional element
which is not supported by ffmpeg's matroska parser. This patch
adds grammar definitions for parsing that element (and few other
related elements) and then puts the data in AVPacket.side_data
with new AVPacketSideDataType AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL.

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -983,6 +983,14 @@ enum AVPacketSideDataType {
      * @endcode
      */
     AV_PKT_DATA_SUBTITLE_POSITION,
+
+    /**
+     * Data found in BlockAdditional element of matroska container. There is
+     * no end marker for the data, so it is required to rely on the side data
+     * size to recognize the end. 8 byte id (as found in BlockAddId) followed
+     * by data.
+     */
+    AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
 };
 
 /**