refactor control and sequencer rendering

src/model/track.rs

@@ -124,7 +124,7 @@ impl Track {
         reset:    bool,
         scope:    &ProcessScope,
         (frame0, frames): (usize, usize),
-        (usec0,  usecs):  (usize, usize),
+        (_usec0,  _usecs):  (usize, usize),
         period:   f64,
     ) {
         // Need to be borrowed outside the conditionals?