avcodec: Define side data type for fallback track

This side data type is meant to be added to AVStream side data.
A fallback track indicates an alternate track to use when the
current track can not be decoded for some reason.  e.g. no
decoder available for codec.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2015-11-xx - xxxxxxx - lavc 57.9.0 - avcodec.h
+  Add AV_PKT_DATA_FALLBACK_TRACK for making fallback associations between
+  streams.
+
 2015-11-xx - xxxxxxx - lavf 57.1.0 - avformat.h
   Add av_stream_new_side_data().
 

libavcodec/avcodec.h

@@ -1129,6 +1129,14 @@ enum AVPacketSideDataType {
      * and FF_LAMBDA_MAX (bad).
      */
     AV_PKT_DATA_QUALITY_FACTOR,
+
+    /**
+     * This side data contains an integer value representing the stream index
+     * of a "fallback" track.  A fallback track indicates an alternate
+     * track to use when the current track can not be decoded for some reason.
+     * e.g. no decoder available for codec.
+     */
+    AV_PKT_DATA_FALLBACK_TRACK,
 };
 
 typedef struct AVPacketSideData {

libavcodec/version.h

@@ -29,7 +29,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR 57
-#define LIBAVCODEC_VERSION_MINOR  8
+#define LIBAVCODEC_VERSION_MINOR  9
 #define LIBAVCODEC_VERSION_MICRO  0
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \