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/GameSrc/Headers/fr3d.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/cit/src/inc/RCS/fr3d.h $
+ * $Revision: 1.2 $
+ * $Author: dc $
+ * $Date: 1994/06/01 22:50:23 $
+ *
+ * Citadel Renderer
+ *  setup and stuff for point counts, initialization, and so on
+ */
+
+//#define FR_PT_CNT   1024
+#define FR_PT_CNT   256
+#define FR_DEF_FOV  80
+#define FR_DEF_AXIS 'X'
+
+#define AXIS_ORDER AXIS_RIGHT, AXIS_DOWN, AXIS_IN
+
+#define ANGLE_ORDER ORDER_YXZ
+#define pitch tx
+#define bank  tz
+#define head  ty
+#define xaxis gX
+#define yaxis gY
+#define zaxis gZ
+
+// conversions
+//  does anyone ever call this... oh yea, for matts stuff...
+//  ick
+#define build_fix_angle(ang) ((65536 * (ang)) / 360)
+
+// coordinate merge - have to inc frcamera.h to use these
+#define coor(val)      (fr_camera_last[val])
+#define ang(val)       (fr_camera_last[val])
+#define last_coor(val) (fr_camera_last[val])
+#define last_ang(val)  (fr_camera_last[val])