Made the set_size() methods of the zoomable_region and scrollable_region

widgets virtual.

dlib/gui_widgets/base_widgets.h

@@ -2189,7 +2189,7 @@ namespace dlib
         virtual ~zoomable_region (
         )= 0;
 
-        void set_pos (
+        virtual void set_pos (
             long x,
             long y
         );
@@ -2231,7 +2231,7 @@ namespace dlib
         double max_zoom_scale (
         ) const;
 
-        void set_size (
+        virtual void set_size (
             unsigned long width,
             unsigned long height
         );
@@ -2436,7 +2436,7 @@ namespace dlib
             long order
         );
 
-        void set_size (
+        virtual void set_size (
             unsigned long width,
             unsigned long height
         );
@@ -2483,7 +2483,7 @@ namespace dlib
             long pos
         );
 
-        void set_pos (
+        virtual void set_pos (
             long x,
             long y
         );

dlib/gui_widgets/base_widgets_abstract.h

@@ -1843,7 +1843,7 @@ namespace dlib
                   (i.e. this is the number that determines how far in the user is allowed to zoom)
         !*/
 
-        void set_size (
+        virtual void set_size (
             unsigned long width,
             unsigned long height
         );
@@ -2039,7 +2039,7 @@ namespace dlib
                   style
         !*/
 
-        void set_size (
+        virtual void set_size (
             unsigned long width,
             unsigned long height
         );