When a window is hidden, its surface and all its roles are destroyed,
if this happens when we already issued a wl_surface_commit and are
awaiting for a frame callback, the clock will remain frozen for the
next time the window is shown.
To avoid this, keep track of the wl_surface_frame() calls issued,
and ensure the clock is thawed after hiding. If we happen to receive
the frame callback, it is just ignored.
https://bugzilla.gnome.org/show_bug.cgi?id=743427
