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.

engine/build_deps.sh

@@ -0,0 +1,72 @@
+#!/bin/bash
+set -e
+
+SDL_version=2.0.9
+SDL2_mixer_version=2.0.4
+
+if [ -d ./build_ext/ ]; then
+	echo A directory named build_ext already exists.
+	echo Please remove it if you want to recompile.
+	exit
+fi
+
+if [ ! -d ./res/ ]; then
+	mkdir ./res/
+fi
+
+mkdir ./build_ext/
+cd ./build_ext/
+
+install_dir=$(pwd)
+
+function build_sdl {
+	curl -O https://www.libsdl.org/release/SDL2-${SDL_version}.tar.gz
+	tar xvf SDL2-${SDL_version}.tar.gz
+	pushd SDL2-${SDL_version}
+
+	./configure --prefix=${install_dir}/built_sdl
+	make
+	make install
+
+	popd
+}
+
+function build_sdl_mixer {
+	curl -O https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-${SDL2_mixer_version}.tar.gz
+	tar xvf SDL2_mixer-${SDL2_mixer_version}.tar.gz
+	pushd SDL2_mixer-${SDL2_mixer_version}
+
+	export SDL2_CONFIG="${install_dir}/built_sdl/bin/sdl2-config"
+	./configure --prefix=${install_dir}/built_sdl_mixer
+	make
+	make install
+
+	popd
+}
+
+function build_fluidsynth {
+	git clone https://github.com/EtherTyper/fluidsynth-lite.git
+	pushd fluidsynth-lite
+	sed -i 's/DLL"\ off/DLL"\ on/' CMakeLists.txt
+	# if building fluidsynth fails, move on without it
+	set +e
+
+	#export CFLAGS="-m32"
+	#export CXXFLAGS="-m32"
+
+	cmake .
+	cmake --build .
+
+	# download a soundfont that's close to the Windows default everyone knows
+	curl -o music.sf2 http://rancid.kapsi.fi/windows.sf2
+	set -e
+	popd
+}
+
+
+build_sdl
+build_sdl_mixer
+build_fluidsynth
+
+cd ..
+mv build_ext/fluidsynth-lite/*.sf2 ./res