Made the integral image swappable

dlib/image_transforms/integral_image.h

@@ -94,12 +94,24 @@ namespace dlib
             return bottom_right - bottom_left - top_right + top_left;
         }
 
+        void swap(integral_image_generic& item)
+        {
+            int_img.swap(item.int_img);
+        }
+
     private:
 
         array2d<T> int_img;
+    };
 
 
-    };
+    template <
+        typename T
+        >
+    void swap (
+        integral_image_generic<T>& a,
+        integral_image_generic<T>& b
+    ) { a.swap(b); }
 
 // ----------------------------------------------------------------------------------------
 

dlib/image_transforms/integral_image_abstract.h

@@ -83,8 +83,27 @@ namespace dlib
                   are contained within the given rectangle.
         !*/
 
+        void swap(
+            integral_image_generic& item
+        );
+        /*!
+            ensures
+                - swaps *this and item
+        !*/
+
     };
 
+// ----------------------------------------------------------------------------------------
+
+    template < typename T >
+    void swap (
+        integral_image_generic<T>& a,
+        integral_image_generic<T>& b
+    ) { a.swap(b); }
+    /*!
+        provides a global swap function
+    !*/ 
+
 // ----------------------------------------------------------------------------------------
 
     typedef integral_image_generic<long> integral_image;