Add accessors for channel data

This is needed when you want to modify the same sample multiple times
within an outer loop.

src/buffer.rs

@@ -123,3 +123,25 @@ impl<'outer, 'inner> Iterator for Channels<'outer, 'inner> {
 }
 
 impl<'outer, 'inner> ExactSizeIterator for Channels<'outer, 'inner> {}
+
+impl<'outer, 'inner> Channels<'outer, 'inner> {
+    /// Access a sample by index. Useful when you would otehrwise iterate over this 'Channels'
+    /// iterator multiple times.
+    #[inline]
+    pub fn get_mut(&mut self, index: usize) -> Option<&mut f32> {
+        // SAFETY: The channel bound has already been checked
+        unsafe {
+            self.buffers
+                .get_unchecked_mut(self.current_channel)
+                .get_mut(index)
+        }
+    }
+
+    /// The same as [Self::get_mut], but without any bounds checking.
+    #[inline]
+    pub unsafe fn get_unchecked_mut(&mut self, index: usize) -> &mut f32 {
+        self.buffers
+            .get_unchecked_mut(self.current_channel)
+            .get_unchecked_mut(index)
+    }
+}