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/shaders/star.frag

@@ -0,0 +1,13 @@
+#version 120
+
+varying float Light;
+
+void main() {
+    vec2 mid = vec2(0.5, 0.5) - gl_PointCoord;
+
+    float dist = 0.5 - length(mid);
+    dist *= 3.0;
+    dist = min(dist, 1.0);
+
+    gl_FragColor = vec4(dist * Light, dist * Light, dist * Light, 1.0);
+}