power: implement AsMilliwatts for slices

src/power.rs

@@ -19,6 +19,12 @@ impl AsMilliwatts for RGB8 {
     }
 }
 
+impl<T> AsMilliwatts for [T] where T: AsMilliwatts {
+    fn as_milliwatts(&self) -> u32 {
+        self.iter().map(|p| { p.as_milliwatts() }).sum()
+    }
+}
+
 pub fn brightness_for_mw(total_mw : u32, target : u8, max_power: u32) -> u8 {
     let target32 = target as u32;
     let requested_mw = (total_mw * target32) / 256;