Convert the game data into editable assets

assets/ now holds every track as a 16-colour indexed PNG (its palette is
the track palette) plus a text file of lap gates and height map, and
every vehicle as a PNG sprite sheet plus a text file with its name and
handling (moved out of the code). The build embeds assets/ instead of
unpacking the ADFs, so the disks are no longer needed to build either.

skids extract converts original disks to assets and checks each file
reads back identically; benchmark results from the assets match the
disks exactly. Assets win over disk data with the same name, and
--data assets picks up edits without a rebuild. assets/README.md
describes how to edit everything.

src/game/mod.rs

@@ -100,7 +100,7 @@ impl Race {
             let ((px, py), heading) = behind(&path, 18.0 + row * 30.0);
             let (rx, ry) = (-heading.sin(), heading.cos());
             let (x, y) = (px + rx * side * 12.0, py + ry * side * 12.0);
-            let car = Car::new(s.vehicle, &vehicles[s.vehicle].id, s.team, x, y, heading);
+            let car = Car::new(s.vehicle, vehicles[s.vehicle].spec, s.team, x, y, heading);
             let line = 0.3 + 0.4 * rand01(&mut rng);
             let pace = 0.86 + 0.12 * rand01(&mut rng);
             entrants.push(Entrant {