Don't expose a temporary grab during a scrollwheel button press/release cycle.

Don't expose a temporary grab during a scrollwheel button press/release cycle.

If we do this there is a short window where we think there is a grab, and
if we generate a client side pointer motion event in this time (e.g. from
a window move) we could be delivering that according to the grab. But in
the Xserver that grab is infinitely short as it sends Press and then
Release immediately.

To see a problem from this, try using the scrollwheel to quickly scroll
in the testgtk list of buttons.