Vendor the Shockolate engine and build it via a Docker image with every

dependency (SDL2, SDL2_mixer, fluidsynth-lite, a MIDI soundfont)
prebuilt, so compiling the engine needs no network access - just the
image and the engine source. Add res/assets/extract_assets.sh to pull
the game's data files out of a purchased GOG installer, and a Makefile
that assembles a runnable dist/ from the two.

Also add `make package`, which builds a redistributable tarball that
omits the proprietary game assets (shipping res/GET_ASSETS.txt instead)
plus license information for both the MIT tooling and the GPLv3 engine,
and a Gitea Actions workflow that builds and publishes it to
dl.ladkau.de on every push.

build-image/build-engine.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# Compiles engine/ (the vendored Shockolate snapshot) against the
+# dependencies prebuilt into this image at /opt/prebuilt - no network
+# access needed. Must be run with the repo root as the working directory
+# - either via ../run-image.sh (which docker-runs this image with the
+# repo bind-mounted at /workspace and WORKDIR set there), or directly
+# when already inside this image (e.g. a CI job using this image as its
+# container - see the Makefile's `engine` target, which picks whichever
+# of these applies).
+#
+# Output lands in engine/.build-output/: the systemshock binary, the
+# shared libraries it needs at runtime (lib/), and a default MIDI
+# soundfont - everything the root Makefile needs to assemble dist/.
+set -euo pipefail
+
+REPO_ROOT="$(pwd)"
+ENGINE_DIR="$REPO_ROOT/engine"
+OUT_DIR="$ENGINE_DIR/.build-output"
+
+cd "$ENGINE_DIR"
+
+echo "== Wiring up prebuilt SDL2/SDL2_mixer/fluidsynth-lite from the image =="
+rm -rf build_ext
+mkdir -p build_ext
+cp -a /opt/prebuilt/built_sdl build_ext/
+cp -a /opt/prebuilt/built_sdl_mixer build_ext/
+cp -a /opt/prebuilt/fluidsynth-lite build_ext/
+
+echo "== Configuring (CMake, BUNDLED SDL2/SDL2_mixer/FluidSynth) =="
+rm -f CMakeCache.txt
+cmake -DENABLE_SDL2=BUNDLED -DENABLE_SOUND=BUNDLED -DENABLE_FLUIDSYNTH=BUNDLED .
+
+echo "== Compiling =="
+make -j"$(nproc)" systemshock
+
+echo "== Assembling engine/.build-output =="
+rm -rf "$OUT_DIR"
+mkdir -p "$OUT_DIR/lib"
+cp systemshock "$OUT_DIR/"
+find build_ext/built_sdl/lib build_ext/built_sdl_mixer/lib build_ext/fluidsynth-lite/src \
+    -name '*.so*' -not -name '*.la' -exec cp -a {} "$OUT_DIR/lib/" \;
+cp /opt/prebuilt/soundfont/default.sf2 "$OUT_DIR/soundfont.sf2"
+
+echo "== Done =="
+echo "Binary:    $OUT_DIR/systemshock"
+echo "Libraries: $OUT_DIR/lib/"
+echo "Soundfont: $OUT_DIR/soundfont.sf2"