layout: Don't update the input region while we have a modal

If we have a modal, the stage's input region doesn't really matter --
all events go to us anyway. To avoid doing extra work doing animations
when we have a modal, like menus, the overview, and the message tray,
just fizzle out all updates.

To make sure we catch updates, update the input region whenever we end a
modal.

https://bugzilla.gnome.org/show_bug.cgi?id=737001