scrolledwindow: set GDK_EXPOSURE_MASK on the overshoot window

scrolledwindow: set GDK_EXPOSURE_MASK on the overshoot window

Currently we use gtk_style_context_set_background() when the state flags
change in order to propagate the background color to the overshoot
window, but this is actually only needed because the window doesn't get
expose events, since we always draw a full background in draw().
This also fixes some problems when the GdkWindow of the scrolled
window's child is composited, as seen in oxygen-gtk3.

https://bugzilla.gnome.org/show_bug.cgi?id=686265