lavc: deprecate refcounted_frames field

No deprecation guards, because the old decode API (for which this field
is needed) doesn't have any either.

This field should be removed together with the old decode calls.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/APIchanges

@@ -13,6 +13,12 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2017-02-01 - xxxxxxx - lavc - avcodec.h
+  Deprecate AVCodecContext.refcounted_frames. This was useful for deprecated
+  API only (avcodec_decode_video2/avcodec_decode_audio4). The new decode APIs
+  (avcodec_send_packet/avcodec_receive_frame) always work with reference
+  counted frames.
+
 2016-xx-xx - xxxxxxx - lavc 57.31.0 - avcodec.h
   Add AVCodecContext.apply_cropping to control whether cropping
   is handled by libavcodec or the caller.

libavcodec/avcodec.h

@@ -2327,7 +2327,7 @@ typedef struct AVCodecContext {
      * - encoding: unused
      * - decoding: set by the caller before avcodec_open2().
      */
-    int refcounted_frames;
+    attribute_deprecated int refcounted_frames;
 
     /* - encoding parameters */
     float qcompress;  ///< amount of qscale change between easy & hard scenes (0.0-1.0)