professionalize peak-meters

The peak meter needs to withstand various test-signals
without visual jitter (in particular 1kHz sine) regardless
of settings (period-size, sample-rate, custom fall-off).

This needs to be done in sync (and not by a random non-rt
‘smoothing’ thread).

On the downside this voids the ‘visual smoothing’ particularly 
with large buffersizes - but then again exactly this “always 
fall-off no matter what [the next real data will be]” is the
problem.

One the upside, there’s one less high-frequency (100Hz) thread
(Yay!) PS. it probably never worked on windows, anyway.

Only peak-meters are affected by his change.
K-meters, IEC I/II and VU were never visually smoothed.

libs/surfaces/control_protocol/control_protocol.cc

@@ -281,7 +281,7 @@ ControlProtocol::route_get_peak_input_power (uint32_t table_index, uint32_t whic
 		return 0.0f;
 	}
 
-	return r->peak_meter().peak_power (which_input);
+	return r->peak_meter().meter_level (which_input, MeterPeak);
 }