Tweak/optimize VST callback:

The audioMasterAutomate callback from plugin to host does include
the parameter-value.

Previously there was a redundant call
Plugin::parameter_changed_externally() -> get_parameter
-> VSTPlugin::get_parameter()   back into the plugin to query the value.

calling back into the plugin from the callback, may explain
oddities and/or crashes with some VSTs.

libs/ardour/ardour/plugin.h

@@ -333,7 +333,7 @@ protected:
 	/* Called when a parameter of the plugin is changed outside of this
 	 * host's control (typical via a plugin's own GUI/editor)
 	 */
-	void parameter_changed_externally (uint32_t which, float val);
+	virtual void parameter_changed_externally (uint32_t which, float val);
 
 	/* should be overridden by plugin API specific derived types to
 	 * actually implement changing the parameter. The derived type should

libs/ardour/ardour/vst_plugin.h

@@ -40,6 +40,7 @@ class PluginInsert;
 class LIBARDOUR_API VSTPlugin : public Plugin
 {
 public:
+	friend class Session;
 	VSTPlugin (AudioEngine &, Session &, VSTHandle *);
 	VSTPlugin (const VSTPlugin& other);
 	virtual ~VSTPlugin ();
@@ -99,6 +100,7 @@ public:
 
 
 protected:
+	void parameter_changed_externally (uint32_t which, float val);
 	void set_plugin (AEffect *);
 	gchar* get_chunk (bool) const;
 	int set_chunk (gchar const *, bool);

libs/ardour/vst_plugin.cc

@@ -137,6 +137,14 @@ VSTPlugin::set_parameter (uint32_t which, float newval)
 	}
 }
 
+void
+VSTPlugin::parameter_changed_externally (uint32_t which, float value )
+{
+	ParameterChangedExternally (which, value); /* EMIT SIGNAL */
+	Plugin::set_parameter (which, value);
+}
+
+
 uint32_t
 VSTPlugin::nth_parameter (uint32_t n, bool& ok) const
 {