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

plugins/plugin-example/src/plugin-main.c

@@ -17,3 +17,8 @@ int FUNC_DECL(EXTERNAL_LIBRARY_INIT_FUNCTION)(const char *params, const char *ve
 	printf("Init with params: %s\ngds-render version: %s\n", params, version);
 	return 0;
 }
+
+int FUNC_DECL(EXTERNAL_LIBRARY_FINALIZE_FUNCTION)(void)
+{
+	return 0;
+}