gdk: Ignore crossings generated by passive grabs when resetting scroll axes

gdk: Ignore crossings generated by passive grabs when resetting scroll axes

Passive grabs may take pointer focus out of the application, even though
the pointer didn't leave the window, but those events still trigger resetting
of the scroll axes. This is most visible with compiz, and possibly other
reparenting WMs, where passive grabs happen on the WM-managed window that
is a parent of the application toplevel.

As it is not possible to have scrolling happening on the timespan a passive
grab takes action, it is entirely safe for GTK+ to assume none happened if
it gets a crossing event of that nature.

https://bugzilla.gnome.org/show_bug.cgi?id=699574#c33