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/track1-6/track.txt

@@ -1,11 +1,14 @@
 # Skids track TRACK1-6. 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 826.15625 521.3125 744.65625 537.3125
 gate 826.0625 647.40625 745.0625 616.65625
 gate 825 776.5 727 680.5
@@ -47,6 +50,7 @@ gate 539.1875 318.125 439.9375 263.375
 gate 535.65625 389.6875 437.78125 406.5625
 gate 576.625 424.75 529.125 502.25
 gate 646.96875 425.4375 648.46875 504.4375
+gate 741.8125 425.625 705.8125 502.625
 
 # Name of the set of tracks this one belongs to, shown in the menu.
 set Racing In The Name Of
@@ -55,8 +59,8 @@ set Racing In The Name Of
 start 434 354
 flags 1
 
-# 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