Add Graphics.loop_at_exit function

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13869 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Patch for PR#5788
(Exception binding fools pattern matching optimisations)

Hopefully it's complete...





git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13871 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02