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/GR/grbm.h

@@ -0,0 +1,53 @@
+/*
+
+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: n:/project/lib/src/2d/RCS/grbm.h $
+ * $Revision: 1.4 $
+ * $Author: kaboom $
+ * $Date: 1993/10/19 10:15:22 $
+ *
+ * Dispatch macros for utility functions.
+ *
+ * This file is part of the 2d library.
+ *
+ * $Log: grbm.h $
+ * Revision 1.4  1993/10/19  10:15:22  kaboom
+ * Now includes tabdat.h.
+ * 
+ * Revision 1.3  1993/10/08  01:15:55  kaboom
+ * Changed quotes in #include liness to angle brackets for Watcom problem.
+ * 
+ * Revision 1.2  1993/06/03  15:10:15  kaboom
+ * Now uses the grd_pixel_table to call bitmap utility functions.
+ * 
+ * Revision 1.1  1993/04/29  18:35:01  kaboom
+ * Initial revision
+ */
+
+#ifndef __GRBM_H
+#define __GRBM_H
+#include "icanvas.h"
+#include "tabdat.h"
+
+#define gr_calc_row(w) \
+   ((short (*)(short _w)) grd_pixel_table[CALC_ROW])(w)
+#define gr_sub_bitmap(bm, x, y, w, h) \
+   ((grs_bitmap *(*)(grs_bitmap *_bm,short _x,short _y,short _w,short _h)) \
+   grd_pixel_table[SUB_BITMAP])(bm, x, y, w, h)
+#endif /* !__GRBM_H */