avutil/frame: Add private_ref to AVFrame

This gives FFmpeg libs a field that they can freely and safely use.
Avoiding the need of wrapping of a users opaque_ref field and its issues.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/frame.h

@@ -563,6 +563,19 @@ typedef struct AVFrame {
     /**
      * @}
      */
+
+    /**
+     * AVBufferRef for internal use by a single libav* library.
+     * Must not be used to transfer data between libraries.
+     * Has to be NULL when ownership of the frame leaves the respective library.
+     *
+     * Code outside the FFmpeg libs should never check or change the contents of the buffer ref.
+     *
+     * FFmpeg calls av_buffer_unref() on it when the frame is unreferenced.
+     * av_frame_copy_props() calls create a new reference with av_buffer_ref()
+     * for the target frame's private_ref field.
+     */
+    AVBufferRef *private_ref;
 } AVFrame;
 
 #if FF_API_FRAME_GET_SET