Reduce the frame interval sample frequency

At the higher frequency we were using earlier, the overhead from
updating the readout contributed significantly to the frame interval.

app-proto/inversive-display/src/main.rs

@@ -101,7 +101,7 @@ fn main() {
         let debug_mode = create_signal(false); /* DEBUG */
         
         /* INSTRUMENTS */
-        const SAMPLE_PERIOD: i32 = 20;
+        const SAMPLE_PERIOD: i32 = 60;
         let mut last_sample_time = 0.0;
         let mut frames_since_last_sample = 0;
         let mean_frame_interval = create_signal(0.0);