avcodec: Undeprecate reordered_opaque

It allows attaching other external, opaque data to the frame and passing it
through the reordering process, for cases when the caller wants other data
than just the plain packet pts.  There is no way to cleanly achieve this
without the field.

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:     2013-12-xx
 
 API changes, most recent first:
 
+2014-08-xx - xxxxxxx - lavc 55.57.3 - avcodec.h
+  reordered_opaque is not going to be removed in the future.
+
 2014-08-xx - xxxxxxx - lavu 53.22.0 - pixfmt.h
   Add AV_PIX_FMT_YA16 pixel format for 16 bit packed gray with alpha.