gdkwindow: Avoid list allocation and object refs during repaint

gdkwindow: Avoid list allocation and object refs during repaint

There is no need to ref the windows we're ignoring, so collect and ref
only the affected child windows. Also, use a on-stack array rather
than allocating a linked list.

Also, we don't need to ref during the event emissions too, as we
already hold a ref.

https://bugzilla.gnome.org/show_bug.cgi?id=754687