Return ignoring of native grab/ungrab events when we don't have a grab

Return ignoring of native grab/ungrab events when we don't have a grab

It turns out we really have to ignore grab/ungrab events or we'll
report double crossing events when we grab or ungrab.

However, we also can't ignore crossing events from grabs from other clients
as that leads to missed enter/leave events on e.g. alt-tab in metacity.

Fortunately we now track grabs very precisely, so we know with certainty
whether we have a grab at the time (serial) of the native crossing events,
and only if we do we ignore them.