support enabling update debugging
support enabling update debugging

If running in debug mode and the GDK_DEBUG_UPDATES environment
variable is set, then enable update debugging on GDK drawables.

This is a useful tool to ensure that no excessive painting is
going on in the app.

arfbtwn/hyena,GNOME/hyena,petejohanson/hyena,petejohanson/hyena,GNOME/hyena,dufoli/hyena,dufoli/hyena,arfbtwn/hyena