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/MusicSrc/MusicDevice.h

@@ -0,0 +1,46 @@
+#pragma once
+
+typedef struct MusicDevice MusicDevice;
+
+typedef enum MusicType
+{
+    Music_None
+   ,Music_AdlMidi
+   ,Music_Native
+#ifdef USE_FLUIDSYNTH
+   ,Music_FluidSynth
+#endif
+} MusicType;
+
+typedef enum MusicMode
+{
+    Music_GeneralMidi,
+    Music_SoundBlaster,
+} MusicMode;
+
+struct MusicDevice
+{
+    int  (*init)(MusicDevice *dev, const unsigned int outputIndex, unsigned samplerate);
+    void (*destroy)(MusicDevice *dev);
+    void (*setupMode)(MusicDevice *dev, MusicMode mode);
+    void (*reset)(MusicDevice *dev);
+    void (*generate)(MusicDevice *dev, short *samples, int numframes);
+    void (*sendNoteOff)(MusicDevice *dev, int channel, int note, int vel);
+    void (*sendNoteOn)(MusicDevice *dev, int channel, int note, int vel);
+    void (*sendNoteAfterTouch)(MusicDevice *dev, int channel, int note, int touch);
+    void (*sendControllerChange)(MusicDevice *dev, int channel, int ctl, int val);
+    void (*sendProgramChange)(MusicDevice *dev, int channel, int pgm);
+    void (*sendChannelAfterTouch)(MusicDevice *dev, int channel, int touch);
+    void (*sendPitchBendML)(MusicDevice *dev, int channel, int msb, int lsb);
+    unsigned int (*getOutputCount)(MusicDevice *dev);
+    void (*getOutputName)(MusicDevice *dev, const unsigned int outputIndex, char *buffer, const unsigned int bufferSize);
+    unsigned short isOpen;    // 1 if device open, 0 if closed
+    unsigned int outputIndex; // index of currently opened output
+    MusicType deviceType;     // type of device
+    char *musicType;          // "sblaster" or "genmidi"
+};
+
+#define MUSICTYPE_SBLASTER "sblaster"
+#define MUSICTYPE_GENMIDI  "genmidi"
+
+MusicDevice *CreateMusicDevice(MusicType type);