+/**
+ * @brief deliberately overlapping memcpy implementation
+ * @param dst destination buffer
+ * @param back how many bytes back we start (the initial size of the overlapping window)
+ * @param cnt number of bytes to copy, must be >= 0
+ *
+ * cnt > back is valid, this will copy the bytes we just copied,
+ * thus creating a repeating pattern with a period length of back.
+ */
+void av_memcpy_backptr(uint8_t *dst, int back, int cnt);
+