widget-tree: Check if the GdkWindow has a native window
widget-tree: Check if the GdkWindow has a native window

GDK_WINDOW_XID forces a native window to be created so we just want to call it
if the GdkWindow actually has a native counterpart. Otherwise we'd get
rendering artifacts on transparent parts of a non-native GdkWindow.

Thanks to Alexander Larsson for pointing out the problem.

GNOME/gtkparasite,GNOME/gtkparasite