image API: increase tolerance for coordinate checks

[bertsky]

Currently, when skew is present (in the annotation of) a page / region, image_from_page / image_from_segment will validate the resulting image has the exact same dimensions as the bounding box of the element after rotation (with an increase in size). But due to rounding of the angle (@orientation) or different methods of binary image rotation, there can always be slight differences, which we should not consider an error:

ERROR ocrd.workspace - page "phys00026" image (binarized,cropped,deskewed; 2579x3717) has not been reshaped properly (2577x3716) during rotation

So we should allow 1 or 2 Pixels tolerance here.