func Rectangle(img *Mat, r image.Rectangle, c color.RGBA, thickness int)
Rectangle draws a simple, thick, or filled up-right rectangle. It renders a rectangle with the desired characteristics to the target Mat image.

For further details, please see: http://docs.opencv.org/master/d6/d6e/group__imgproc__draw.html#ga346ac30b5c74e9b5137576c9ee9e0e8c