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/Clip/clprect.c

@@ -0,0 +1,80 @@
+/*
+
+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/clprect.c $
+ * $Revision: 1.4 $
+ * $Author: kaboom $
+ * $Date: 1993/10/19 09:50:14 $
+ * 
+ * Routines for clipping rectangles to a rectangle.
+ *
+ * This file is part of the 2d library.
+ *
+ * $Log: clprect.c $
+ * Revision 1.4  1993/10/19  09:50:14  kaboom
+ * Replaced #include <grd.h" with new headers split from grd.h.
+ * 
+ * Revision 1.3  1993/10/01  15:40:23  kaboom
+ * Converted to include clpcon.h instead of clip.h.
+ * 
+ * Revision 1.2  1993/05/04  17:33:59  kaboom
+ * Changed to deal with new padded clipping regions.
+ * 
+ * Revision 1.1  1993/02/22  14:41:56  kaboom
+ * Initial revision
+ */
+
+#include "clpcon.h"
+#include "cnvdat.h"
+
+/* clips the top, left, right, and bottom edges of a rectangle to the current
+   clip rectangle.  pointers to the values are passed in so the values can be
+   modified.  returns the clip code. */
+int gr_clip_rect (short *left, short *top, short *right, short *bot)
+{
+   int code = CLIP_NONE;
+
+   if (*right<=grd_clip.left || *left>=grd_clip.right ||
+       *bot<=grd_clip.top  || *top>=grd_clip.bot)
+      /* rect is completely clipped. */
+      return CLIP_ALL;
+
+   if (*left < grd_clip.left) {
+      /* rect is off the left edge of the window. */
+      *left = grd_clip.left;
+      code |= CLIP_LEFT;
+   }
+   if (*right > grd_clip.right) {
+      /* off the right edge of the window. */
+      *right = grd_clip.right;
+      code |= CLIP_RIGHT;
+   }
+   if (*top < grd_clip.top) {
+      /* off the top of the window. */
+      *top = grd_clip.top;
+      code |= CLIP_TOP;
+   }
+   if (*bot > grd_clip.bot) {
+      /* off the bottom of the window. */
+      *bot = grd_clip.bot;
+      code |= CLIP_BOT;
+   }
+
+   return code;
+}