Allow Lua DSP scripts to override strict-i/o

This is handy for 1in, 2out plugins or 2in, 1in out (and multiples
thereof).

libs/ardour/ardour/luaproc.h

@@ -85,6 +85,7 @@ public:
 	void cleanup () { }
 
 	int set_block_size (pframes_t /*nframes*/) { return 0; }
+	bool connect_all_audio_outputs () const { return _connect_all_audio_outputs; }
 
 	int connect_and_run (BufferSet& bufs,
 			samplepos_t start, samplepos_t end, double speed,
@@ -153,6 +154,7 @@ private:
 	std::string _docs;
 	bool _lua_does_channelmapping;
 	bool _lua_has_inline_display;
+	bool _connect_all_audio_outputs;
 
 	void queue_draw () { QueueDraw(); /* EMIT SIGNAL */ }
 	DSP::DspShm lshm;

libs/ardour/luaproc.cc

@@ -60,6 +60,7 @@ LuaProc::LuaProc (AudioEngine& engine,
 	, _script (script)
 	, _lua_does_channelmapping (false)
 	, _lua_has_inline_display (false)
+	, _connect_all_audio_outputs (false)
 	, _designated_bypass_port (UINT32_MAX)
 	, _signal_latency (0)
 	, _control_data (0)
@@ -254,6 +255,26 @@ LuaProc::load_script ()
 		_lua_latency = new luabridge::LuaRef (lua_dsp_latency);
 	}
 
+	/* parse I/O options */
+	luabridge::LuaRef ioconfig = luabridge::getGlobal (L, "dsp_ioconfig");
+	if (ioconfig.isFunction ()) {
+		try {
+			luabridge::LuaRef iotable = ioconfig ();
+			if (iotable.isTable ()) {
+				for (luabridge::Iterator i (iotable); !i.isNil (); ++i) {
+					if (!i.key().isString()) {
+						continue;
+					}
+					if (i.key().cast<std::string> () == "connect_all_audio_outputs" && i.value().isBoolean ()) {
+						_connect_all_audio_outputs = i.value().cast<bool> ();
+					}
+				}
+			}
+		} catch (...) {
+			return true;
+		}
+	}
+
 	// initialize the DSP if needed
 	luabridge::LuaRef lua_dsp_init = luabridge::getGlobal (L, "dsp_init");
 	if (lua_dsp_init.type () == LUA_TFUNCTION) {