surface: Refine airport texture and model.
surface: Make Aerial AI not increase throttle when touching ground.

Previously, we determined if a vehicle is touching the ground by its
orientation and velocity, which is not really correct.
We have monitored touching status since taxiing capability is
introduced.  We can use the status to judge if we're really touching
ground (in Bullet Dynamics World).
surface: Make Aerial's friction be affected gradually.

The friction has been affected only by spoiler flag, and it's only
updated when spoiler property is assigned from a Squirrel script.
This commit make the friction affected by fspoiler to reflect its state
in real time.
surface: Make A10's auto pilot be able to taxi to destination.

The auto pilot AI has been able to fly around destination position when
flying, but haven't been able to on the ground.
This commit enables A10 auto pilot to taxi on the ground, using throttle
and rudder.  Unlike flying AI, it will stop when reached the destination.
surface: Experiment with taxi path for A10.

Also tweaked arrival determination radius and steering sensitivity
to look good.