Create masks_to_bboxes that returns tight bounding boxes

[wkentaro]

Why?

• xmin, ymin will be consistent to xmax, ymax
• can easily clip by np.clip.
• bboxes = [[ymin, xmin, ymax, xmax], ...]

width, height = image.shape[:2]

ymin = bboxes[:, 0]
xmin = bboxes[:, 1]
ymax = bboxes[:, 2]
xmax = bboxes[:, 3]

ymin = np.clip(ymin, 0, height - 1)
ymax = np.clip(ymax, 0, height - 1)
xmin = np.clip(xmin, 0, width - 1)
xmax = np.clip(xmax, 0, width - 1)