And get the embedder before continuing, as the backend-specific
get_monitor_at_window () only knows about its own windows.

In order to check for offscreen windows, we introduce a new function
gdk_window_is_impl_offscreen (). Unlike gdk_window_is_offscreen (),
it doesn't rely on the window_type field, but actually checks whether
GDK_WINDOW_IMPL_GET_CLASS (window->impl) is GdkOffscreenWindow.

See previous commit for informations.