Fix crash on exit for some client apps.
Fix crash on exit for some client apps.

Hard to control destruction order in an app which doesn't keep
tuvok around for its entire runtime.  Thus we don't want to use
the singleton accessor in functions which might be called during
destruction.

git-svn-id: 83c4c97093aeea62ac982fbfd516bf9db27f5c99@1271 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok