During resizing we froze window updates when configuring the
window, and unfroze the window updates when processing the
next resize. This wasn't absolutely reliable, because we might
not have a next resize. Instead tie window freezing more
directly to the current sync request value - a window is
frozen until it catches up with the last value we sent it
in _NET_WM_SYNC_REQUEST.

Testing with unresponsive clients showed that there was a bug
where window->disable_sync once set, would not actually disable
sync, but it *would* disable noticing that the client was
unresponsive for the next resize. Fix that by checking for
->disable_sync before sending _NET_WM_SYNC_REQUEST.

https://bugzilla.gnome.org/show_bug.cgi?id=694046