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.

(cherry picked from commit ed033b1c)