Use the original computer drivers, car collisions and towing

Decoded from SKIDMARKS_CD32:

- Computer drivers: the path of nodes laid 2 game units apart along
  the middle of the road, with side points 1 unit in from each wall.
  Drivers aim four nodes past where they'll be in four frames, or cut
  to the other side two nodes on at 70% throttle when running wide.
  Replaces the pure-pursuit AI; laps are quicker on every track.
- Collisions: six points on the player's car (corners and sides, from
  .set words 6 and 7, now known to be half-length and half-width) are
  tested against the other car's box. Hits push the cars apart with a
  strength that grows with their speed difference, and spin the car.
  As in the original, only human players' cars collide; cars at
  different heights don't.
- Towing: the hitch is at the car's back, the caravan sits 0.656 units
  behind it, and past 90 degrees both car and caravan bend back.

The reconstructed collision weight (`mass`) is gone; old asset files
with it still load.

assets/vehicles/mini.txt

@@ -9,9 +9,8 @@ trailer no
 # 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.
+#   3 steering, 5 gravity, 7 and 8 half the car's length and width
+#   (for collisions; the tow hitch is at the back). 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 56 24 30 12 3 24 20 12 46 10 100 0 0 0 0 0
-# weight when cars collide (not from the original data)
-mass 0.8