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/src/Libraries/2D/Source/Flat8/fl8g24.c

@@ -0,0 +1,71 @@
+/*
+
+Copyright (C) 2015-2018 Night Dive Studios, LLC.
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+/*
+ * $Source: r:/prj/lib/src/2d/RCS/fl8g24.c $
+ * $Revision: 1.4 $
+ * $Author: kevin $
+ * $Date: 1994/10/17 14:59:58 $
+ *
+ * Routines for reading 24-bit pixels from a flat 8 canvas.
+ *
+ * This file is part of the 2d library.
+ *
+ * $Log: fl8g24.c $
+ * Revision 1.4  1994/10/17  14:59:58  kevin
+ * Use palette macros in preparation for switch to palette globals.
+ *
+ * Revision 1.3  1993/10/19  09:50:22  kaboom
+ * Replaced #include <grd.h" with new headers split from grd.h.
+ *
+ * Revision 1.2  1993/10/01  15:44:52  kaboom
+ * Converted to include clpcon.h instead of clip.h.
+ *
+ * Revision 1.1  1993/09/02  20:03:41  kaboom
+ * Initial revision
+ */
+
+#include "clpcon.h"
+#include "cnvdat.h"
+#include "scrdat.h"
+
+/* set an unclipped pixel in bank-switched memory. */
+long flat8_get_upixel24(short x, short y) {
+    uchar *p;
+    long *r;
+    int i;
+
+    p = grd_canvas->bm.bits + grd_canvas->bm.row * y + x;
+    i = *p;
+    r = (long *)(grd_pal + 3 * i);
+    return *r & 0x00ffffff;
+}
+
+/* set a clipped pixel in bank-switched memory.  return the clip code. */
+long flat8_get_pixel24(short x, short y) {
+    uchar *p;
+    long *r;
+    int i;
+
+    if (x < grd_clip.left || x >= grd_clip.right || y < grd_clip.top || y >= grd_clip.bot)
+        return CLIP_ALL;
+    p = grd_canvas->bm.bits + grd_canvas->bm.row * y + x;
+    i = *p;
+    r = (long *)(grd_pal + 3 * i);
+    return *r & 0x00ffffff;
+}