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.

assets/vehicles/midget.txt

@@ -0,0 +1,16 @@
+# Skids vehicle settings. Lines starting with # are comments.
+# The sprite sheet is midget.png: 32 headings across, 25 tilts down.
+name MIDGET
+# top speed on tarmac, world pixels per second
+top 205
+# acceleration and braking, world pixels per second squared
+accel 185
+brake 320
+# sideways grip (higher = less sliding)
+grip 4
+# steering rate, radians per second
+turn 4
+# weight in collisions
+mass 0.85
+# speed and grip multiplier off the road
+offroad 0.6