app: properly shut down gimp-parallel in stable versions

In app_exit_after_callback(), call gimp_gegl_exit() before
gegl_exit() when performing a quick shut-down in stable versions,
so that gimp-parallel, and, in particular, the async thread pool,
is properly shut down.  Code running in the async thread pool may
use GEGL (in particular, now the drawable previews are rendered
asynchronously), and calling gegl_exit() while it's still running
is unsafe.

app/app.c

@@ -504,6 +504,8 @@ app_exit_after_callback (Gimp       *gimp,
 
 #else
 
+  gimp_gegl_exit (gimp);
+
   gegl_exit ();
 
   exit (EXIT_SUCCESS);