window: Make sure not to respond to input events on OR windows

This can happen since we select for events on the root window, and
clients themselves might not select for input, meaning the X server
will bubble up. Just do nothing and ignore the event in this case.

This should hopefully fix some of the

Window manager warning: Log level 8: meta_window_raise: assertion '!window->override_redirect' failed
Window manager warning: Log level 8: meta_window_focus: assertion '!window->override_redirect' failed

spam that people have been seeing.