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/MacSrc/Modding.h

@@ -0,0 +1,52 @@
+/*
+
+Copyright (C) 2018-2020 Shockolate Project
+
+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/>.
+
+*/
+
+#ifndef __MODDING_H
+#define __MODDING_H
+
+//--------------------
+// Defines
+//--------------------
+
+#define MAX_MOD_FILES 100
+
+//--------------------
+// Public Globals
+//--------------------
+
+// Let people override the default game archive
+extern char *modding_archive_override;
+
+// Additional resource files to load
+extern char *modding_additional_files[MAX_MOD_FILES];
+
+extern int num_mod_files;
+
+//--------------------
+//  Function Prototypes
+//--------------------
+
+int AddResourceFile(char *filename);
+int AddArchiveFile(char *filename);
+int ProcessModFile(char *filename, uchar follow_dirs);
+int ProcessModArgs(int argc, char **argv);
+int ProcessModDirectory(char *dirname);
+int LoadModFiles(void);
+
+#endif //__MODDING_H