frame clock: schedule in refresh intervals

When the application does not receive "frame drawn" signals we schedule
the clock to run more or less at intervals equal to the last known
refresh interval. In order to minimize clock skew we have to aim for
exact intervals.

(cherry picked from commit f5de46670b)

gdk/gdkframeclockidle.c

@@ -562,7 +562,7 @@ gdk_frame_clock_paint_idle (void *data)
   if (priv->freeze_count == 0)
     {
       priv->min_next_frame_time = compute_min_next_frame_time (clock_idle,
-                                                               priv->frame_time);
+                                                               priv->smoothed_frame_time_base);
       maybe_start_idle (clock_idle, FALSE);
     }