Don't allow to deactivate meter.

before this change:

1) switch to 'custom' meter point,
2) deactivate meter processor.

-> meters does not run regardless of meter-point
-> meter can only be re-nabled in 'custom' mode

libs/ardour/ardour/meter.h

@@ -75,6 +75,9 @@ public:
 	/** Compute peaks */
 	void run (BufferSet& bufs, framepos_t start_frame, framepos_t end_frame, pframes_t nframes, bool);
 
+	void activate ()   { }
+	void deactivate () { }
+
 	ChanCount input_streams () const { return current_meters; }
 	ChanCount output_streams () const { return current_meters; }
 

libs/ardour/meter.cc

@@ -43,6 +43,8 @@ PeakMeter::PeakMeter (Session& s, const std::string& name)
 	Iec1ppmdsp::init(s.nominal_frame_rate());
 	Iec2ppmdsp::init(s.nominal_frame_rate());
 	Vumeterdsp::init(s.nominal_frame_rate());
+	_pending_active = true;
+	_active = true;
 }
 
 PeakMeter::~PeakMeter ()