Switch to an extended form of _NET_WM_SYNC_REQUEST_COUNTER

Switch to an extended form of _NET_WM_SYNC_REQUEST_COUNTER

By exporting two XSync counters on a toplevel window, we subscribe
to an extended form of the _NET_WM_SYNC_REQUEST_COUNTER protocol,
where the window manager can initiate an atomic frame, as previously,
but the application can also do so by incrementing the new counter to
an odd value, and then to an even value to finish the frame.

See:
https://mail.gnome.org/archives/wm-spec-list/2011-October/msg00006.html

The support for 64-bit integers that GLib requires is used to
simplify the logic.

https://bugzilla.gnome.org/show_bug.cgi?id=685460