postprocessor.py: fix NumPy array issue when using latest opencv release

[Hulongyin]

This PR is to fix the NumPy array marked as readonly issue in postprocessor.py when using the latest opencv release.

When using the latest opencv release, the postprocess function in postprocessor.py will raise, thus causing the inference service crash:

RuntimeError: Error during postprocessing: OpenCV(4.9.0) :-1: error: (-5:Bad argument) in function 'rectangle'
> Overload resolution failed:
>  - img marked as output argument, but provided NumPy array marked as readonly
>  - Expected Ptr<cv::UMat> for argument 'img'
>  - img marked as output argument, but provided NumPy array marked as readonly
>  - Expected Ptr<cv::UMat> for argument 'img'

This is related to the opencv patch opencv/opencv#24026 which fixed opencv/opencv-python#859.

We need to set the WRITEABLE flag of frame array to true before cv2.rectangle, and the flag cannot be successfully set by using setflags function, which will raiseValueError: cannot set WRITEABLE flag to True of this array
So we use frame = frame.copy() to set WRITEABLE flag.

[leyao-daily]

LGTM. Thansk