app: App: run(): fix: receive from frame only if currently playing

Prior to this, once paused, would get stuck at receiving the frame
since the receiver sleeps when nothing is available, thus no events
are polled. By receiving only when playing, this eliminates the issue.

NOTE: cannot use `try_recv()` since it would lead to flickering when
this thread is running faster than the audio producing thread.

src/app.rs

···
         let mut last_poll = Instant::now();
 
         loop {
-            let audio_frame = self.frame_rx.recv().unwrap();
-            let channels =
-                stream_to_matrix(audio_frame.data.iter().cloned(), audio_frame.channels, 1.);
+            let channels = (!self.graph.pause)
+                .then(|| self.frame_rx.recv().unwrap())
+                .map(|audio_frame| {
+                    stream_to_matrix(audio_frame.data.iter().cloned(), audio_frame.channels, 1.)
+                });
 
             fps += 1;
 
···
                         datasets.append(&mut current_display.references(&graph));
                     }
                 }
-                if let Some(current_display) = self.current_display_mut() {
+                if let Some((current_display, channels)) = self.current_display_mut().zip(channels)
+                {
                     datasets.append(&mut current_display.process(&graph, &channels));
                 }
                 terminal