Fixed Fl_Device destructor (made it virtual). This eliminates the compiler

"warning: 'class Fl_Device' has virtual functions but non-virtual destructor"
on Mac OS X.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@8529 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

FL/Fl_Device.H

@@ -86,6 +86,13 @@ public:
    \endcode
    */
   virtual const char *class_name() {return class_id;};
+  /** 
+   Virtual destructor.
+   
+   The destructor of Fl_Device must be virtual to make the destructors of
+   derived classes being called correctly on destruction.
+   */
+  virtual ~Fl_Device() {};
 };
 
 #define FL_REGION_STACK_SIZE 10