Fix bugs in darknet_video.py

[yamahitsuji]

I found darknet_video.py don't save output video correctly and fixed 2 bugs.

1. cv2.VideoWriter frame size
   cv2.VideoWriter frame size must be original video size (not resized image size).
2. converting rgb to bgr
   A image is already bgr.

[AlexeyAB]

@yamahitsuji Thanks!

[SatyaNekkanti]

Hi @yamahitsuji

• I was trying to figure out why do we need to invert the color band of a video frame here https://github.com/AlexeyAB/darknet/blob/master/darknet_video.py#L113
• OpenCV VideoCapture function directly loads the video frame with True Color band(RGB) whereas OpenCV loads images in the inverted color band (BGR) Please correct me if I am wrong.
• Does the invert of color band mess up with darknet detections?