fix spatial shift for bboxes 2d

[Aurelien-VB]

All actual changes are in the _spatial_shift function, everything else comes from the line wrapping which doesn't appear to be at the same format.

The issue was coming from the xcyc format and then clamp that would move the center and thus keep boxes that are outside the image.
Instead I just use the xyxy format making it work directly.

I'm not sure I understand why we did a min with ones before, I replaced everything with the clamp.

[Aurelien-VB]

I also added a fix to frame spatial shift because when the shift was negative there was pixels at the border not being masked out.