lavc: deprecate AV_CODEC_FLAG_DROPCHANGED

This decoding flag makes decoders drop all frames after a parameter
change, but what exactly constitutes a parameter change is not well
defined and will typically depend on the exact use case.
This functionality then does not belong in libavcodec, but rather in
user code

libavcodec/avcodec.h

@@ -226,11 +226,15 @@ typedef struct RcOverride{
  * Use qpel MC.
  */
 #define AV_CODEC_FLAG_QPEL            (1 <<  4)
+#if FF_API_DROPCHANGED
 /**
  * Don't output frames whose parameters differ from first
  * decoded frame in stream.
+ *
+ * @deprecated callers should implement this functionality in their own code
  */
 #define AV_CODEC_FLAG_DROPCHANGED     (1 <<  5)
+#endif
 /**
  * Request the encoder to output reconstructed frames, i.e.\ frames that would
  * be produced by decoding the encoded bistream. These frames may be retrieved
@@ -2713,9 +2717,6 @@ int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);
  *                          no more output frames
  * @retval AVERROR(EINVAL)  codec not opened, or it is an encoder without the
  *                          @ref AV_CODEC_FLAG_RECON_FRAME flag enabled
- * @retval AVERROR_INPUT_CHANGED current decoded frame has changed parameters with
- *                          respect to first decoded frame. Applicable when flag
- *                          AV_CODEC_FLAG_DROPCHANGED is set.
  * @retval "other negative error code" legitimate decoding errors
  */
 int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);