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/InitMac.h

@@ -0,0 +1,46 @@
+/*
+
+Copyright (C) 1994-1995 Looking Glass Technologies, Inc.
+Copyright (C) 2015-2018 Night Dive Studios, LLC.
+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/>.
+
+*/
+// InitMac.h - Initialize Mac toolbox managers and setup the application's globals.
+
+// How many tick passed since game startup
+extern uint32_t gShockTicks;
+
+// Pointer to screen
+extern intptr_t *gScreenAddress;
+extern int32_t gScreenRowbytes;
+// Size of current window
+extern int32_t gScreenWide, gScreenHigh;
+
+//--------------------
+//  Prototypes
+//--------------------
+
+/// Initialize the Macintosh managers.
+void InitMac(void);
+
+/**
+ * Normal cleanup when the program quits.
+ * @deprecated Actually does nothing
+ */
+void CleanupAndExit(void);
+
+/// Startup the SystemShock timer.
+void InstallShockTimers(void);