make handles with their handlevisibility property set to "callback" visible in callback functions * graphics.cc (executing_callback): New file scope variable. (callback_property::execute): Protect executing_callback and set it to true when executing callback function. * graphics.cc (base_properties::is_handle_visible): Move here from graphics.h.in. Check executing_callback to decide whether handles that have their handlevisibility property set to "callback" should be visible.

--- a/src/graphics.h.in	Tue Oct 11 21:55:21 2011 -0400
+++ b/src/graphics.h.in	Tue Oct 11 22:02:39 2011 -0400
@@ -2382,10 +2382,7 @@
   virtual bool is_climinclude (void) const { return false; }
   virtual bool is_aliminclude (void) const { return false; }
 
-  bool is_handle_visible (void) const
-  {
-    return ! handlevisibility.is ("off");
-  }
+  bool is_handle_visible (void) const;
 
   std::set<std::string> dynamic_property_names (void) const;