avformat/avio{,buf}: deprecate AVIOContext::written

Originally added as a private entry in commit
3f75e5116b, but its grouping with
the comment noting its private state was missed during merging of
the field from Libav (most likely due to an already existing field
in between).

libavformat/avio.h

@@ -290,7 +290,13 @@ typedef struct AVIOContext {
      */
     int ignore_boundary_point;
 
+#if FF_API_AVIOCONTEXT_WRITTEN
+    /**
+     * @deprecated field utilized privately by libavformat.
+     */
+    attribute_deprecated
     int64_t written;
+#endif
 
     /**
      * Maximum reached position before a backward seek in the write buffer,