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.c

@@ -20,6 +20,7 @@
 #include <stdint.h>
 #include <string.h>
 
+#include "avassert.h"
 #include "buffer_internal.h"
 #include "common.h"
 #include "mem.h"
@@ -355,3 +356,10 @@ AVBufferRef *av_buffer_pool_get(AVBufferPool *pool)
 
     return ret;
 }
+
+void *av_buffer_pool_buffer_get_opaque(AVBufferRef *ref)
+{
+    BufferPoolEntry *buf = ref->buffer->opaque;
+    av_assert0(buf);
+    return buf->opaque;
+}