Add finalize function as exported function to external renderer and use it in the main app after rendering

include/gds-render/output-renderers/external-renderer-interfaces.h

@@ -32,6 +32,15 @@
  */
 #define EXTERNAL_LIBRARY_INIT_FUNCTION exported_init
 
+/**
+ * @brief Function name expected to be found in external library for finalizing.
+ *
+ * @code
+ * int EXTERNAL_LIBRARY_FINALIZE_FUNCTION(void);
+ * @endcode
+ */
+#define EXTERNAL_LIBRARY_FINALIZE_FUNCTION exported_finalize
+
 /**
  * @brief Global integer specified by an external renderer to signal, that the init and render functions shall be executed in a subprocess
  *