license_plate_detection_yunet: fix NMSBoxes input (use [x,y,w,h] from 4-point corners)

[Hariom-Nagar211]

What

Fix NMSBoxes usage in lpd_yunet.py.
Previously, it passed [x1, y1, x2, y2] (corner coords), but cv.dnn.NMSBoxes expects [x, y, w, h].

How

• Convert the 4-point quadrilateral into axis-aligned [x, y, w, h] rectangles only for NMS.
• Use returned indices (keepIdx) to filter the original quadrilateral detections.
• Normalize keepIdx across OpenCV versions (tuple vs array).

Why

• Prevents incorrect suppression during NMS.
• Keeps original quadrilateral boxes intact for downstream usage.
• Resolves issue license_plate_detection_yunet NMSBoxes call with wrong bboxes #275.

Notes

• Tested with sample images: overlapping plates are suppressed correctly, and detections remain quadrilateral-shaped.