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.

docs/FORMATS.md

@@ -130,6 +130,9 @@ These almost certainly live on the missing disk 1:
 - Sound effects and music.
 - Any tracks or vehicles beyond the 24 and 8 above.
 
+The engine's own editable formats, which `skids extract` converts all of
+the above into, are described in [assets/README.md](../assets/README.md).
+
 If disk 1 turns up, put it next to the others and run `skids info`.
 Anything in a `track/` or `vehicles/` directory inside its archive is
 picked up automatically.