Coordinate backward conversion

Hi,

I am wondering if this part will introduces some bugs `https://github.com/DetectionTeamUCAS/R2CNN_Faster-RCNN_Tensorflow/blob/854fe56697132ef82b13e9570f077fcd9e6c6e78/libs/box_utils/coordinate_convert.py#L43`

Because cv2 minRectArea gives us angles ranging from -90 to 0 degrees, which means some box will be rotated 90 degrees if we use this `back_forward_convert` func. Is this expected?

Any suggestion on fixing this?

Best,
Xin