surface: Make hitboxes of SurfaceBuilding represent hit shape with Bullets.

Previously, they only indicate Bullet dynamics engine shape.  To make
it more realistic, we need to make them Bullet ray trace hit shapes, too.
surface: Make AI controlled A10 and F15 stop on the ground.

Previously, they always try to fly no matter where they are, assuming
there's no ground.  Now they just stop increasing throttle if on a
ground.

This behavior is useful for keeping AI controlled planes on an airport.

They're not really detecting ground, but just look velocity and body
orientation to determine whether it look like on a ground.
surface: Spoiler implementation to F15 and A10.
surface: Re-model airport to have some structures.
surface: Erased no longer used landing gear switch in Aerial::control().
surface: Enabled taxiing capability to Aerial on the ground.

You can control steering direction by rudder.
surface: Make A10's shape parameters easier to take off.
surface: Make F15's shape parameters easier to take off.