When events are paused, we should not return TRUE from prepare() or check().
GTK+ handles this for events that are already in the GTK+ queue, but
we also need suppress checks for events that are in the system queue - if we
return TRUE indicating that there are events in the system queue, then we'll
call dispatch(), and do nothing. The event source will spin, and will never
run the other phases of the paint clock.

(Broadway doesn't have a window system queue separate from the GDK event queue,
but we write the function the same way for consistency.)

https://bugzilla.gnome.org/show_bug.cgi?id=694274