Fixed the gaussian() function used by the SURF code. It wasn't

computing a properly weighted Gaussian function.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403070

dlib/image_keypoint/surf.h

@@ -29,7 +29,7 @@ namespace dlib
             << "\n\t sig: " << sig 
         );
         const double pi = 3.1415926535898;
-        return 1.0/(sig*std::sqrt(2*pi)) * std::exp( -(x*x + y*y)/(2*sig*sig));
+        return 1.0/(sig*sig*2*pi) * std::exp( -(x*x + y*y)/(2*sig*sig));
     }
 
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