AIDockShip could use tag_landing

[mike-f1]

Because tag_landing are added to models, then you need to use them
when you use autopilot.

This let autopilot use them because GetLandingPosOffset() return
tag_landing.y if it's present, else return GetAabb().min.y as usual
(check ship.cpp:280)

Before merge this, there's a need to tweak ships collision geometry
to reach the eight of tag_landing

Note: seems my editor do lot of changes, but the only interesting line
is 966...

[mike-f1]

#3910

#3901