enchancement: allow to construct CvMat from native cv::Mat pointer.

[solganik]

Description

There are some cases when mix of native code and golang code are used. For example in use-case i have control path is implemented in golang and video processing pipeline implemented in native. In my use-case pipeline uses opencv directly and provides a callback to golang code with cv::mat.
Now in opencv there is a cv::mat copy constructor that creates a cv::mat header, and increments a recfounter on the data.
Look here: https://docs.opencv.org/4.5.2/d3/d63/classcv_1_1Mat.html#a294eaf8a95d2f9c7be19ff594d06278e for more details of cvmat copy constructor.

I implemented the patch in described in #864 to allow re-use of native cv:mat without data copy.