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_win64.sh

@@ -0,0 +1,107 @@
+#!/bin/bash
+set -e
+
+SDL_version=2.0.10
+SDL2_mixer_version=2.0.4
+GLEW_version=2.1.0
+CMAKE_target=Unix\ Makefiles
+
+# Removing the mwindows linker option lets us get console output
+function remove_mwindows {
+	sed -i -e "s/ \-mwindows//g" Makefile
+}
+
+function build_sdl {
+	curl -O https://www.libsdl.org/release/SDL2-devel-${SDL_version}-mingw.tar.gz
+	tar xvf SDL2-devel-${SDL_version}-mingw.tar.gz
+	cp -r SDL2-${SDL_version}/x86_64-w64-mingw32/ built_sdl/
+}
+
+function build_sdl_mixer {
+	curl -O https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-devel-${SDL2_mixer_version}-mingw.tar.gz
+	tar xf SDL2_mixer-devel-${SDL2_mixer_version}-mingw.tar.gz --exclude=Xcode
+	cp -r SDL2_mixer-${SDL2_mixer_version}/x86_64-w64-mingw32/ built_sdl_mixer/
+}
+
+function build_glew {
+	curl -O https://netcologne.dl.sourceforge.net/project/glew/glew/${GLEW_version}/glew-${GLEW_version}.tgz
+	tar xvf glew-${GLEW_version}.tgz
+	mv glew-${GLEW_version}/ built_glew/
+	pushd built_glew
+	mingw32-make glew.lib
+	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
+	cmake -G "${CMAKE_target}" .
+	cmake --build .
+
+	# download a soundfont that's close to the Windows default everyone knows
+	curl -o music.sf2 http://rancid.kapsi.fi/windows.sf2
+	set -e
+	popd
+}
+
+## Actual building starts here
+
+if [ -d ./build_ext/ ]; then
+	echo A directory named build_ext already exists.
+	echo Please remove it if you want to recompile.
+	exit
+fi
+
+if ! [ -x "$(command -v cmake)" ]; then
+	echo CMake is needed to install Shockolate. 
+	echo Please download CMake from https://cmake.org/download/,
+	echo install it and try again in a new Git Bash window.
+	exit
+fi
+
+rm -rf CMakeFiles/
+rm -rf CMakeCache.txt
+
+cp windows/make.exe /usr/bin/
+
+if [ ! -d ./res/ ]; then
+mkdir ./res/
+fi
+
+mkdir ./build_ext/
+cd ./build_ext/
+install_dir=`pwd -W`
+
+
+build_sdl
+build_sdl_mixer
+build_glew
+build_fluidsynth
+
+
+# Back to the root directory, copy required DLL files for the executable
+cd ..
+cp build_ext/built_sdl/bin/SDL*.dll .
+cp build_ext/built_sdl_mixer/bin/SDL*.dll .
+cp build_ext/built_glew/lib/*.dll .
+cp build_ext/fluidsynth-lite/src/*.dll .
+
+# move the soundfont to the correct place if we successfully built fluidsynth
+mv build_ext/fluidsynth-lite/*.sf2 ./res
+
+# Set up build.bat
+if [[ -z "${APPVEYOR}" ]]; then
+	echo "Normal build"
+	echo "@echo off
+	cmake -G \"${CMAKE_target}\" .
+	mingw32-make systemshock" >build.bat
+else
+	echo "Appveyor"
+	echo "cmake -G \"${CMAKE_target}\" . 
+	make systemshock" >build.bat
+fi
+
+echo "Our work here is done. Run BUILD.BAT in a Windows shell to build the actual source."