Not thread safe

[Lecrapouille]

Hi! This line

Multithreading-OpenCV-CPP/src/WebcamStream.cpp

Line 73 in 02f28f1

return &this->frame;

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;
}