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
This is specifically about managing X11 windows, not necessarily
running as an X11 compositor. By that I mean that this code is
still used for XWayland windows, and event handling is still and
modesetting / monitor management is still in core/.
This is also a fairly conservative move. We don't move anything
like screen.c or bell.c in here, even though those are really
only for X11 clients.
