The meat of xnestUpdateModifierState was ifdef'd out in
6ef46c40e6. This resulted in stuck modifiers
when a modifier key release event wasn't sent to Xnest (e.g. Alt-Tab away).
See X.Org Bug 3664 <https://bugs.freedesktop.org/show_bug.cgi?id=3664> for
the original bug report.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>