cv::compare's output does not match documentation (Bug #3149)

Commit d8417af0860 ([1], July 2011) changed cv::compare to operate on n-channel matrices. The function's destination is now also n-channel. The documentation² was not updated accordingly and still describes the result to be CV_8UC1. Getting a single-channel output is useful: it can be fed into cv::countNonZero to determine whether two matrices are exactly equal. cv::countNonZero does not work with matrices with more than one channel.

The documentation should be updated to reflect the changes made in commit d8417af0860, and there should be a way to retrieve a single-channel comparison matrix and/or cv::countNonZero should be extended to support n-channel matrices.

Example code:

const auto
  mat1 = cv::Mat::zeros(100, 50, CV_8UC4),
  mat2 = cv::Mat::ones(100, 50, CV_8UC4);

const auto tmp = cv::Mat(mat1 != mat2);

std::cout << "channels = " << tmp.channels() << std::endl;
std::cout << "nonzero = " << cv::countNonZero(mat1 != mat2) << std::endl;

Output:

channels = 4
OpenCV Error: Assertion failed (src.channels() == 1 && func != 0) in countNonZero, file /[…]/modules/core/src/stat.cpp, line 496

[1] https://github.com/Itseez/opencv/commit/d8417af0860b8c718722734e5898ddaea055699e
[2] http://docs.opencv.org/modules/core/doc/operations_on_arrays.html#compare