bug fix: qr code crop

qr code was failing due to this bug.
now fixed and tested with 30 qr codes from google images, all but one
failed, and redlaser app didn't do better.
Note: redlaser has their own qr code segmentation algorithms. So for
some qr code placed in black background, we'd fail and they wouldn't.
But if you place it in a white background with sufficient padding, ours
won't fail either