window-props: _NET_WM_WINDOW_OPACITY range is 0xffffffff to 0

After reading the atom, scale the value from 0xffffffff to 0xff. Not doing so
causes Clutter to truncate the opacity value, and only read the last two digits.
Hence, 0x7fffffff (50%) becomes 0xff (100%).

https://bugzilla.gnome.org/show_bug.cgi?id=727874