Not thread safe

[Lecrapouille]

Hi! This line

https://github.com/AriNguyen/Multithreading-OpenCV-CPP/blob/02f28f1a287544810df1457043aa186610ba797d/src/WebcamStream.cpp#L73

is not thread safe. I guess a fix is:

cv::Mat WebcamStream::read() {
    const std::lock_guard<[std::mutex](https://en.cppreference.com/w/cpp/thread/mutex)> lock(m);
    return this->frame;
}