osx: Add windowDidChangeScreen delegate

Added -windowDidChangeScreen: delegate to handle condition where moving
host window to a different screen would cause pick errors to be output.
The delegate just causes the stage to be redrawn which re-creates the
pick buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=655306

clutter/osx/clutter-stage-osx.c

@@ -131,6 +131,11 @@ clutter_stage_osx_get_wrapper (ClutterStageWindow *stage_window);
   else 
     return [self frame].size;
 }
+
+- (void)windowDidChangeScreen:(NSNotification *)notification
+{
+  clutter_redraw(CLUTTER_STAGE(self->stage_osx->wrapper));
+}
 @end
 
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