Fix: Clear track_handles when SetTrack heard (#270)

Clear `track_handles` when `SetTrack` heard, to prevent an issue where
indices of `tracks` become desynced with those of `track_handles`.

Closes #232.

src/driver/tasks/mixer/mod.rs

@@ -231,6 +231,7 @@ impl Mixer {
             MixerMessage::AddTrack(t) => self.add_track(t),
             MixerMessage::SetTrack(t) => {
                 self.tracks.clear();
+                self.track_handles.clear();
 
                 let mut out = self.fire_event(EventMessage::RemoveAllTracks);