Embed the game data so the disks are not needed at run time

build.rs unpacks the ADF images next to Cargo.toml (or in
SKIDS_EMBED_DIR) with the same ADF/LHA readers the game uses and packs
the files into the executable. At run time the built-in data loads
first and anything given with --data, or found next to the executable,
overrides it. The embed feature is on by default; builds without it
behave as before and ask for --data. CI now runs the binary from an
empty directory.

Cargo.toml

@@ -8,10 +8,13 @@ readme = "README.md"
 repository = "https://github.com/andydixon/skids"
 
 [features]
-default = ["audio"]
+default = ["audio", "embed"]
 # Procedural engine/skid sound via cpal. Disable with --no-default-features
 # on Linux machines without the ALSA development headers.
 audio = ["dep:cpal"]
+# Build the game data from the ADF images next to Cargo.toml (or in
+# SKIDS_EMBED_DIR) into the executable, so it runs without the disks.
+embed = []
 
 [dependencies]
 minifb = "0.28"