Fix erasing of arrow highlight (XB)

Fix erasing of arrow highlight (XB)

On large board with long-range oblique moves (like in Grande Acedrex)
the head of the arrow was not always erased (and not always drawn either).
This was partly due to the arrow pointing to a corner of the target square
even if the path was nearly orthogonal, while the damage trail is always
assumed to connect the square centers. Having the arrow point more
precisely and widening the damage trail a bit solved the problem.