Use the original handling, walls and projection from the executable

Decoded from SKIDMARKS_CD32's machine code:

- Car physics: the per-frame engine/grip/steering update, gravity and
  jumps, the hill effect on the throttle, and body tilt. Replaces the
  reconstructed handling model.
- The meaning of every .set value (separate drag, grip and power for
  human and computer drivers; steering, gravity, tow hitch), and the
  six handling presets, offered by a new Handling menu option.
- Walls: the road edges are quadratic B-splines through the gates'
  ends. They match the painted road edges.
- The real projection, which lifts points by their height; the height
  map is indexed by world cells, not screen pixels.
- Tracks have one more gate than the stored count: the last gate was
  being dropped on every track.

Computer drivers now follow the road between the walls and steer like
the original's. Vehicle assets store the values as `handling`; older
files still load. Docs describe the decoded logic.

assets/vehicles/caravan.txt

@@ -4,20 +4,14 @@ name CARAVAN
 driver SIMON
 # yes = towed behind cars (Caravans option) instead of driven
 trailer yes
-# The 16 values from the original .set file. Their meaning is not
-# decoded yet, so the game does not use them; the handling below is
-# a reconstruction.
-original 2 65532 65532 1 12 10 16 14 0 0 0 0 0 0 0 0
-# top speed on tarmac, world pixels per second
-top 200
-# acceleration and braking, world pixels per second squared
-accel 170
-brake 320
-# sideways grip (higher = less sliding)
-grip 5
-# steering rate, radians per second
-turn 3.6
-# weight in collisions
+#
+# handling: the 16 values from the original .set file, used as the
+# original game does. Counting from 1:
+#   1 2 6    computer drivers: drag, grip, power
+#   9 10 11  human drivers: drag, grip, power
+#   3 steering, 5 gravity, 7 tow hitch distance; the rest are unused.
+# Top speed = power * drag / (64 - drag) / 32 world units per frame,
+# 50 frames a second (drag below 64). Higher grip, less sliding.
+handling 2 65532 65532 1 12 10 16 14 0 0 0 0 0 0 0 0
+# weight when cars collide (not from the original data)
 mass 1
-# speed and grip multiplier off the road
-offroad 0.55