Fixed a bug in the image_window and image_display GUI tools which caused them

to not redraw overlay lines correctly in certain cases involving non-default
zoom levels.

dlib/gui_widgets/widgets.cpp

@@ -5794,10 +5794,8 @@ namespace dlib
         // push this new overlay into our overlay vector
         overlay_lines.push_back(overlay);
 
-        const point origin(total_rect().tl_corner());
-
         // make the parent window redraw us now that we changed the overlay
-        parent.invalidate_rectangle(rectangle(overlay.p1+origin, overlay.p2+origin));
+        parent.invalidate_rectangle(get_rect_on_screen(rectangle(overlay.p1, overlay.p2)));
     }
 
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