device: add RMSMeter

crates/device/src/meter.rs

@@ -0,0 +1,16 @@
+use crate::*;
+
+#[derive(Debug, Default, Clone)]
+pub struct RMSMeter(f32);
+
+impl RMSMeter {
+    pub fn set (&mut self, samples: &[u32]) {
+        let sum: usize = samples.iter().map(|s|*s as usize).reduce(|sum, sample|sum + sample)
+            .unwrap_or(0);
+        self.0 = (sum as f32 / samples.len() as f32).sqrt();
+    }
+}
+
+render!(TuiOut: |self: RMSMeter, to| {
+    let [x, y, w, h] = to.area();
+});