Use less arbitrary decay weights for the gain GUIs

This solves the same problem as #27 but in a cleaner way. The previous
commits refactored the smoothing to make it possible to use the
calculations in plugin code to do the same thing as #27, but then I
realized that that doesn't make much sense since the time to decay into
complete silence isn't very meaningful for this kind of computation.

plugins/examples/gain_gui_vizia/src/lib.rs

@@ -5,6 +5,9 @@ use std::sync::Arc;
 
 mod editor;
 
+/// The time it takes for the peak meter to decay by 12 dB after switching to complete silence.
+const PEAK_METER_DECAY_MS: f64 = 150.0;
+
 /// This is mostly identical to the gain example, minus some fluff, and with a GUI.
 pub struct Gain {
     params: Arc<GainParams>,
@@ -99,8 +102,11 @@ impl Plugin for Gain {
         buffer_config: &BufferConfig,
         _context: &mut impl InitContext,
     ) -> bool {
-        // TODO: How do you tie this exponential decay to an actual time span?
-        self.peak_meter_decay_weight = 0.9992f32.powf(44_100.0 / buffer_config.sample_rate);
+        // After `PEAK_METER_DECAY_MS` milliseconds of pure silence, the peak meter's value should
+        // have dropped by 12 dB
+        self.peak_meter_decay_weight = 0.25f64
+            .powf((buffer_config.sample_rate as f64 * PEAK_METER_DECAY_MS / 1000.0).recip())
+            as f32;
 
         true
     }