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.

res/run.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Launches System Shock. Copied next to the systemshock binary in dist/
+# by the root Makefile.
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")"
+export LD_LIBRARY_PATH="$(pwd)/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+exec ./systemshock "$@"