Do not modify input parameter in MergeDebevec

MergeDebevec takes camera response function as an optional input
parameter. Despite being clearly marked as input, this matrix is
overwritten during processing.

This commit adds a temporary matrix to store the log response and avoid
modification of the input.

modules/photo/src/merge.cpp

@@ -79,9 +79,11 @@ public:
             response = linearResponse(channels);
             response.at<Vec3f>(0) = response.at<Vec3f>(1);
         }
-        log(response, response);
-        CV_Assert(response.rows == LDR_SIZE && response.cols == 1 &&
-                  response.channels() == channels);
+
+        Mat log_response;
+        log(response, log_response);
+        CV_Assert(log_response.rows == LDR_SIZE && log_response.cols == 1 &&
+                  log_response.channels() == channels);
 
         Mat exp_values(times);
         log(exp_values, exp_values);
@@ -103,7 +105,7 @@ public:
             w /= channels;
 
             Mat response_img;
-            LUT(images[i], response, response_img);
+            LUT(images[i], log_response, response_img);
             split(response_img, splitted);
             for(int c = 0; c < channels; c++) {
                 result_split[c] += w.mul(splitted[c] - exp_values.at<float>((int)i));