In case of an offscreen window find its onscreen embedder, and NULL
any GdkWindowImpl that's not quartz, checking all return values.

This replaces 16ded683, removing its unnecessary search_for_nearest
functions.

Any unchecked cast of a GdkWindowImpl to GdkWindowImplQuartz risks at
least getting an invalid member reference, not just the ones in
gdk_quartz_window_get_foo, and all calls to gdk_quartz_window_get_foo need
to be checked, not just Gtk's internal calls. The motivation was that
transient_for was getting set on a Gimp offscreen window and that caused a
crash in raise_transient.