title: opencv.js meanStdDev implementation
body: <p>I am using opencv.js to get average color from a masked region, the API I am using is cv.meanStdDev. The code:</p>
  <pre><code>            let mask = cv.Mat.zeros(image.cols, image.rows, cv.CV_8UC3);
              let mean = new cv.Mat;
              let std = new cv.Mat;
  
              cv.meanStdDev(image, mean, std);
  </code></pre>
  <p>is good, but when I add the last 'mask' parameter:</p>
  <pre><code>            cv.meanStdDev(image, mean, std, mask);
  </code></pre>
  <p>in browser console it prints:</p>
  <pre><code>Uncaught 6577936
  </code></pre>
  <p>And the script stopped. Seems like the implementation of meanStdDev is broken?</p>