pax-britannica: Fix Ship collisionCenter calculation position error.

regression introduced by d08ff17f2519c17883f1234de0afdf086da7cc3f

Signed-off-by: Xerxes Rånby <xerxes@zafena.se>
Merge pull request #1193 from xranby/master

pax-britannica: Fix Ship collisionCenter calculation position error.