avutil/buffer: add av_buffer_pool_buffer_get_opaque

In order to access the original opaque parameter of a buffer in the buffer
pool. (The buffer pool implementation overrides the normal opaque parameter but
also saves it so it is accessible).

v2: add assertion check before dereferencing the BufferPoolEntry.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavutil/buffer.h

@@ -284,6 +284,19 @@ void av_buffer_pool_uninit(AVBufferPool **pool);
  */
 AVBufferRef *av_buffer_pool_get(AVBufferPool *pool);
 
+/**
+ * Query the original opaque parameter of an allocated buffer in the pool.
+ *
+ * @param ref a buffer reference to a buffer returned by av_buffer_pool_get.
+ * @return the opaque parameter set by the buffer allocator function of the
+ *         buffer pool.
+ *
+ * @note the opaque parameter of ref is used by the buffer pool implementation,
+ * therefore you have to use this function to access the original opaque
+ * parameter of an allocated buffer.
+ */
+void *av_buffer_pool_buffer_get_opaque(AVBufferRef *ref);
+
 /**
  * @}
  */