Added another overload of add_overlay(), this one doesn't require a pixel type

and just makes the overlay red.

dlib/gui_widgets/widgets.h

@@ -3679,6 +3679,9 @@ namespace dlib
         void add_overlay(const rectangle& r, pixel_type p) 
         { add_overlay(image_display::overlay_rect(r,p)); }
 
+        void add_overlay(const rectangle& r) 
+        { add_overlay(image_display::overlay_rect(r,rgb_pixel(255,0,0))); }
+
         template <typename pixel_type>
         void add_overlay(const rectangle& r, pixel_type p, const std::string& l) 
         { add_overlay(image_display::overlay_rect(r,p,l)); }
@@ -3694,6 +3697,9 @@ namespace dlib
             add_overlay(temp);
         }
 
+        void add_overlay(const std::vector<rectangle>& r) 
+        { add_overlay(r, rgb_pixel(255,0,0)); }
+
         void add_overlay(
             const full_object_detection& object,
             const std::vector<std::string>& part_names

dlib/gui_widgets/widgets_abstract.h

@@ -2917,7 +2917,7 @@ namespace dlib
         template <typename pixel_type>
         void add_overlay(
             const rectangle& r, 
-            pixel_type p
+            pixel_type p = rgb_pixel(255,0,0)
         );
         /*!
             ensures
@@ -2938,7 +2938,7 @@ namespace dlib
         template <typename pixel_type>
         void add_overlay(
             const std::vector<rectangle>& r,
-            pixel_type p
+            pixel_type p = rgb_pixel(255,0,0)
         );
         /*!
             ensures