fix wrong union calculation in box_overlap_testing.h (#2735)

Co-authored-by: Moosbauer, Sebastian <sebastian.moosbauer@posteo.de>

dlib/image_processing/box_overlap_testing.h

@@ -20,8 +20,8 @@ namespace dlib
         const double inner = a.intersect(b).area();
         if (inner == 0)
             return 0;
-        const double outer = (a+b).area();
-        return inner/outer;
+        const double outer = a.area() + b.area() - inner;
+        return inner / outer;
     }
 
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