Fixes frames getting out of order due to constant span_length

The `current_span_length` is in samples not frames. Rodio was using a fixed
span_length for all sources regardless of their number of channels. If
that span length was not wholly dividable by the number of channels it
would create an offset.

Credit for finding this goes to: @will3942

Author: dvdsk

src/queue.rs

@@ -117,7 +117,6 @@ pub struct SourcesQueueOutput<S> {
     input: Arc<SourcesQueueInput<S>>,
 }
 
-const THRESHOLD: usize = 512;
 impl<S> Source for SourcesQueueOutput<S>
 where
     S: Sample + Send + 'static,
@@ -143,7 +142,7 @@ where
                 && self.input.next_sounds.lock().unwrap().is_empty()
             {
                 // The next source will be a filler silence which will have the length of `THRESHOLD`
-                return Some(THRESHOLD);
+                return Some(self.silent_span_length());
             }
         }
 
@@ -156,7 +155,7 @@ where
         }
 
         // Otherwise we use the constant value.
-        Some(THRESHOLD)
+        Some(self.silent_span_length())
     }
 
     #[inline]
@@ -234,7 +233,10 @@ where
             let mut next = self.input.next_sounds.lock().unwrap();
 
             if next.len() == 0 {
-                let silence = Box::new(Zero::<S>::new_samples(1, 44100, THRESHOLD)) as Box<_>;
+                // queue reports number of channels for the current source. Not the silence source.
+                // `self.silent_span_length` accounts for this.
+                let silence =
+                    Box::new(Zero::<S>::new_samples(1, 44100, self.silent_span_length())) as Box<_>;
                 if self.input.keep_alive_if_empty.load(Ordering::Acquire) {
                     // Play a short silence in order to avoid spinlocking.
                     (silence, None)
@@ -250,6 +252,11 @@ where
         self.signal_after_end = signal_after_end;
         Ok(())
     }
+
+    /// 200 frames of silence
+    fn silent_span_length(&self) -> usize {
+        200 * self.channels() as usize
+    }
 }
 
 #[cfg(test)]