core: Add clear function to (Raw)SampleBuffer.

Addresses #126.

symphonia-core/src/audio.rs

@@ -735,6 +735,11 @@ impl<S: Sample> SampleBuffer<S> {
         self.buf.len()
     }
 
+    /// Clears all written samples.
+    pub fn clear(&mut self) {
+        self.n_written = 0;
+    }
+
     /// Copies all audio data from the source `AudioBufferRef` in planar channel order into the
     /// `SampleBuffer`. The two buffers must be equivalent.
     pub fn copy_planar_ref(&mut self, src: AudioBufferRef)
@@ -1005,6 +1010,11 @@ impl<S: Sample + RawSample> RawSampleBuffer<S> {
         self.buf.len()
     }
 
+    /// Clears all written samples.
+    pub fn clear(&mut self) {
+        self.n_written = 0;
+    }
+
     /// Gets an immutable slice to the bytes of the sample's written in the `RawSampleBuffer`.
     pub fn as_bytes(&self) -> &[u8] {
         // Get a slice to the written raw samples in the buffer, and convert from &[RawType] to