Add multi-channel mask support to mean, meanStdDev and setTo

This adds the possibility to use multi-channel masks for the functions
cv::mean, cv::meanStdDev and the method Mat::setTo. The tests have now a
probability to use multi-channel masks for operations that support them.
This also includes Mat::copyTo, which supported multi-channel masks
before, but there was no test confirming this.

modules/core/include/opencv2/core.hpp

@@ -608,7 +608,7 @@ CV_EXPORTS_W void findNonZero( InputArray src, OutputArray idx );
 
 The function cv::mean calculates the mean value M of array elements,
 independently for each channel, and return it:
-\f[\begin{array}{l} N =  \sum _{I: \; \texttt{mask} (I) \ne 0} 1 \\ M_c =  \left ( \sum _{I: \; \texttt{mask} (I) \ne 0}{ \texttt{mtx} (I)_c} \right )/N \end{array}\f]
+\f[\begin{array}{l} N_c =  \sum _{I: \; {\texttt{mask} (I)_c} \ne 0} 1 \\ M_c =  \left ( \sum _{I: \; {\texttt{mask} (I)_c} \ne 0}{ \texttt{src} (I)_c} \right )/N_c \end{array}\f]
 When all the mask elements are 0's, the function returns Scalar::all(0)
 @param src input array that should have from 1 to 4 channels so that the result can be stored in
 Scalar_ .
@@ -622,7 +622,7 @@ CV_EXPORTS_W Scalar mean(InputArray src, InputArray mask = noArray());
 The function cv::meanStdDev calculates the mean and the standard deviation M
 of array elements independently for each channel and returns it via the
 output parameters:
-\f[\begin{array}{l} N =  \sum _{I, \texttt{mask} (I)  \ne 0} 1 \\ \texttt{mean} _c =  \frac{\sum_{ I: \; \texttt{mask}(I) \ne 0} \texttt{src} (I)_c}{N} \\ \texttt{stddev} _c =  \sqrt{\frac{\sum_{ I: \; \texttt{mask}(I) \ne 0} \left ( \texttt{src} (I)_c -  \texttt{mean} _c \right )^2}{N}} \end{array}\f]
+\f[\begin{array}{l} N_c =  \sum _{I, {\texttt{mask} (I)_c}  \ne 0} 1 \\ \texttt{mean} _c =  \frac{\sum_{ I: \; {\texttt{mask} (I)_c} \ne 0} \texttt{src} (I)_c}{N_c} \\ \texttt{stddev} _c =  \sqrt{\frac{\sum_{ I: \; {\texttt{mask} (I)_c} \ne 0} \left ( \texttt{src} (I)_c -  \texttt{mean} _c \right )^2}{N_c}} \end{array}\f]
 When all the mask elements are 0's, the function returns
 mean=stddev=Scalar::all(0).
 @note The calculated standard deviation is only the diagonal of the

modules/core/include/opencv2/core/mat.hpp

@@ -1192,8 +1192,8 @@ public:
     /** @overload
     @param m Destination matrix. If it does not have a proper size or type before the operation, it is
     reallocated.
-    @param mask Operation mask. Its non-zero elements indicate which matrix elements need to be copied.
-    The mask has to be of type CV_8U and can have 1 or multiple channels.
+    @param mask Operation mask of the same size as \*this. Its non-zero elements indicate which matrix
+    elements need to be copied. The mask has to be of type CV_8U and can have 1 or multiple channels.
     */
     void copyTo( OutputArray m, InputArray mask ) const;
 
@@ -1229,7 +1229,8 @@ public:
 
     This is an advanced variant of the Mat::operator=(const Scalar& s) operator.
     @param value Assigned scalar converted to the actual array type.
-    @param mask Operation mask of the same size as \*this.
+    @param mask Operation mask of the same size as \*this. Its non-zero elements indicate which matrix
+    elements need to be copied. The mask has to be of type CV_8U and can have 1 or multiple channels
      */
     Mat& setTo(InputArray value, InputArray mask=noArray());