Ask LuaProc to drop references

This fixes a circular shared_ptr<> reference that prevents
plugin destruction.

LuaProc may hold references to Route that contains the plugin
or the PluginInsert of the LuaPlugin. These are only dropped
when the interpreter collects garbage.

Previously this happened in the d'tor or LuaProc, but while the
Plugin has a reference to the Insert, the Insert is not deleted
and the d'tor is never called.

libs/ardour/ardour/luaproc.h

@@ -78,6 +78,8 @@ public:
 
 	PluginOutputConfiguration possible_output () const { return _output_configs; }
 
+	void drop_references ();
+
 	std::set<Evoral::Parameter> automatable() const;
 
 	void activate () { }

libs/ardour/ardour/plugin.h

@@ -106,6 +106,8 @@ public:
 		activate ();
 	}
 
+	virtual void drop_references () {}
+
 	virtual std::set<Evoral::Parameter> automatable () const                   = 0;
 	virtual std::string                 describe_parameter (Evoral::Parameter) = 0;
 	virtual std::string                 state_node_name () const               = 0;

libs/ardour/ardour/plugin_insert.h

@@ -61,6 +61,8 @@ public:
 	PluginInsert (Session&, boost::shared_ptr<Plugin> = boost::shared_ptr<Plugin>());
 	~PluginInsert ();
 
+	void drop_references ();
+
 	static const std::string port_automation_node_name;
 
 	int set_state(const XMLNode&, int version);

libs/ardour/luaproc.cc

@@ -130,7 +130,7 @@ LuaProc::~LuaProc () {
 				_stats_max[1] * (float)_stats_cnt / _stats_avg[1]);
 	}
 #endif
-	lua.do_command ("collectgarbage();");
+	lua.collect_garbage ();
 	delete (_lua_dsp);
 	delete (_lua_latency);
 	delete [] _control_data;
@@ -182,6 +182,12 @@ LuaProc::init ()
 	lua.do_command ("function ardour () end");
 }
 
+void
+LuaProc::drop_references ()
+{
+	lua.collect_garbage ();
+}
+
 boost::weak_ptr<Route>
 LuaProc::route () const
 {

libs/ardour/plugin_insert.cc

@@ -110,6 +110,15 @@ PluginInsert::~PluginInsert ()
 	}
 }
 
+void
+PluginInsert::drop_references ()
+{
+	for (Plugins::iterator i = _plugins.begin(); i != _plugins.end(); ++i) {
+		(*i)->drop_references ();
+	}
+	Processor::drop_references ();
+}
+
 void
 PluginInsert::set_strict_io (bool b)
 {