avcodec: add AVHWAccel.free_frame_priv callback

libavcodec/decode.c

@@ -1718,3 +1718,23 @@ int ff_copy_palette(void *dst, const AVPacket *src, void *logctx)
     }
     return 0;
 }
+
+AVBufferRef *ff_hwaccel_frame_priv_alloc(AVCodecContext *avctx,
+                                         const AVHWAccel *hwaccel)
+{
+    AVBufferRef *ref;
+    AVHWFramesContext *frames_ctx = (AVHWFramesContext *)avctx->hw_frames_ctx->data;
+    uint8_t *data = av_mallocz(hwaccel->frame_priv_data_size);
+    if (!data)
+        return NULL;
+
+    ref = av_buffer_create(data, hwaccel->frame_priv_data_size,
+                           hwaccel->free_frame_priv,
+                           frames_ctx->device_ctx, 0);
+    if (!ref) {
+        av_free(data);
+        return NULL;
+    }
+
+    return ref;
+}