VST3: Re-activate plugin when latency changes

This is required. From the "RestartFlags" documentation:

> kLatencyChanged:
>   Latency has changed The plug informs the host that its latency
>   has changed, getLatencySamples should return the new latency after
>   setActive (true) was called The host has to deactivate and reactivate
>   the plug-in, then afterwards the host could ask for the current
>   latency (getLatencySamples) see IAudioProcessor::getLatencySamples

libs/ardour/vst3_plugin.cc

@@ -1247,6 +1247,9 @@ VST3PI::restartComponent (int32 flags)
 		update_shadow_data ();
 	}
 	if (flags & Vst::kLatencyChanged) {
+		/* need to re-activate the plugin as per spec */
+		deactivate ();
+		activate ();
 		_plugin_latency.reset ();
 	}
 	return kResultOk;