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/tracks/track4-2/track.txt

@@ -1,11 +1,14 @@
 # Skids track TRACK4-2. Lines starting with # are comments.
 #
 # The picture is track.png: 1024x512, 16 colours. Its palette is the
-# track's palette. Colours 8-15 are road (full grip), 0-7 are scenery.
+# track's palette. Colours 8-15 are road, 0-7 are scenery (dusty).
 #
 # gate x1 y1 x2 y2: lap gates in race order, in world units. Gate 0 is
-# the start/finish line; the grid forms up behind it. World to picture:
-#   picture x = wx - wy + 512, picture y = (wx + wy) * 31/64 - 252
+# the start/finish line; the grid forms up behind it. The gates' ends
+# also shape the walls: each edge of the road is a smooth curve
+# through the matching ends of the gates. World to picture, for a
+# point at height z (pixels):
+#   picture x = wx - wy + 512, picture y = (wx + wy) / 2 - 156 - z
 gate 495.78125 950.1875 540.1875 849.78125
 gate 400.03125 867.9375 513.3125 821.65625
 gate 374.78125 805.1875 508.4375 791.53125
@@ -46,6 +49,7 @@ gate 711.6875 750.40625 583.65625 688.46875
 gate 715.375 842.65625 586.40625 755
 gate 701.84375 891.1875 585.65625 814.03125
 gate 663.3125 939.1875 581.0625 837.78125
+gate 608.78125 967.1875 571.4375 852.53125
 
 # Name of the set of tracks this one belongs to, shown in the menu.
 set Sounf Of Speed
@@ -54,8 +58,8 @@ set Sounf Of Speed
 start 122 216
 flags 0
 
-# height: 64 rows of 64 values, ground height x 256. Each value covers
-# 16x8 pixels of the picture.
+# height: 64 rows (world y) of 64 values (world x), ground height in
+# pixels x 64. Each value covers 16 x 16 world units.
 height
 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400
 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400 6400