decode: add a per-frame private data for hwaccel use

This will be useful in the CUVID hwaccel. It should also eventually
replace current decoder-specific mechanisms used by various other
hwaccels.

libavcodec/decode.c

@@ -1025,6 +1025,9 @@ static void decode_data_free(void *opaque, uint8_t *data)
     if (fdd->post_process_opaque_free)
         fdd->post_process_opaque_free(fdd->post_process_opaque);
 
+    if (fdd->hwaccel_priv_free)
+        fdd->hwaccel_priv_free(fdd->hwaccel_priv);
+
     av_freep(&fdd);
 }
 

libavcodec/decode.h

@@ -49,6 +49,12 @@ typedef struct FrameDecodeData {
     int (*post_process)(void *logctx, AVFrame *frame);
     void *post_process_opaque;
     void (*post_process_opaque_free)(void *opaque);
+
+    /**
+     * Per-frame private data for hwaccels.
+     */
+    void *hwaccel_priv;
+    void (*hwaccel_priv_free)(void *priv);
 } FrameDecodeData;
 
 /**