Changed the image_window so it doesn't try to adjust the size or position

when set_image() is called with an image the same size as the one currently
being displayed.

dlib/gui_widgets/widgets.h

@@ -3599,6 +3599,12 @@ namespace dlib
             auto_mutex M(wm);
             gui_img.set_image(img); 
 
+            // Only ever mess with the size of the window if the user is giving us an image
+            // that is a different size.  Otherwise we assume that they will have already
+            // sized the window to whatever they feel is reasonable for an image of the
+            // current size.  
+            if (previous_image_size != get_rect(img))
+            {
                 const rectangle r = gui_img.get_image_display_rect();
                 if (image_rect != r)
                 {
@@ -3610,6 +3616,8 @@ namespace dlib
 
                     image_rect = r;
                 }
+                previous_image_size = get_rect(img);
+            }
         }
 
         void add_overlay (
@@ -3765,6 +3773,7 @@ namespace dlib
 
         image_display gui_img;
         rectangle image_rect;
+        rectangle previous_image_size;
         bool window_has_closed;
         bool have_last_click;
         point last_clicked_point;