Count inside edge of border as true box, not the center of the border

[antingshen]

Currently, since the border size is a few pixels, the boxes stored in the annotation are 0.5 * border_width larger than they actually are, assuming annotators are using the inside of the border to inscribe objects (right?).

We can fix this by either accounting for the offset in the box itself, or just convert all the boxes when we save the annotation.