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/main.vert

@@ -0,0 +1,19 @@
+#version 110
+
+attribute vec2 texcoords;
+attribute float light;
+attribute vec4 color;
+
+varying vec2 TexCoords;
+varying float Light;
+varying vec4 Color;
+
+uniform mat4 view;
+uniform mat4 proj;
+
+void main() {
+    TexCoords = texcoords;
+    Light = light;
+    Color = color;
+    gl_Position = proj * view * gl_Vertex;
+}