Handle corner case where WinRT speech that doesn't interrupt, and is played after a delay, causes recently-spoken utterances to replay.

`MediaPlayer` only seems to have states for playing and paused, but not stopped. Further, playing when the queue is finished seems to restart playback from the beginning.

Here we clear the list of items to play if the player is paused and we're on the last item. We assume we're done with all items to speak, and clear the list before appending a new item and beginning playback again.

The correct solution is probably to investigate how events work in winrt-rs, but callbacks and Rust have always been a disaster when I've tried them, so I'm hesitant. This does seem to handle the basic scenarios I've thrown at it.

src/backends/winrt.rs

@@ -56,6 +56,14 @@ impl Backend for WinRT {
         let content_type = stream.content_type()?;
         let source = MediaSource::create_from_stream(stream, content_type)?;
         let item = MediaPlaybackItem::create(source)?;
+        let state = self.player.playback_session()?.playback_state()?;
+        if state == MediaPlaybackState::Paused {
+            let index = self.playback_list.current_item_index()?;
+            let total = self.playback_list.items()?.size()?;
+            if index == total - 1 {
+                self.playback_list.items()?.clear()?;
+            }
+        }
         self.playback_list.items()?.append(item)?;
         if !self.is_speaking()? {
             self.player.play()?;