cogl: reset pending swaps counter on unrealize

When removing the frame callback on the CoglOnscreen, we loose the ability
to get notified of swap events. This could leave us with a counter != 0
which leads to a deadlock situation after the next realize/draw cycle.

https://bugzilla.gnome.org/show_bug.cgi?id=755014

clutter/cogl/clutter-stage-cogl.c

@@ -77,6 +77,8 @@ clutter_stage_cogl_unrealize (ClutterStageWindow *stage_window)
       cogl_object_unref (stage_cogl->onscreen);
       stage_cogl->onscreen = NULL;
     }
+
+  stage_cogl->pending_swaps = 0;
 }
 
 static void