Do not send VisibilityNotify events when MapUnmapEvents are disabled

This prevents a protocol visible side-effect (XVisibilityEvent) on
XCompositeRedirectWindow() followed by a XCompositeUnredirectWindow().

The problem shows up in gnome-screensaver with compiz and "unredirect
fullscreen windows" enable. A VisibilityNotify event is generated (first
with obscured and than with unobscured) when the window swithces from
redirected to unredirected.

https://bugs.freedesktop.org/show_bug.cgi?id=18133
http://launchpad.net/bugs/278112

dix/window.c

@@ -3091,6 +3091,8 @@ void
 SendVisibilityNotify(WindowPtr pWin)
 {
     xEvent event;
+    if (!MapUnmapEventsEnabled(pWin))
+        return;
 #ifndef NO_XINERAMA_PORT
     unsigned int visibility = pWin->visibility;
 #endif