Show tiles for members we're trying to connect to

This should help give more context on what's going wrong in
splitbrain scenarios.

If users leave calls uncleanly, their tile will remain in until
their member event times out, which will be an hour from when they
joined the call. See https://github.com/vector-im/element-call/issues/639.

Part of https://github.com/vector-im/element-call/issues/616

src/video-grid/useMediaStream.ts

@@ -35,21 +35,31 @@ declare global {
 export const useMediaStreamTrackCount = (
   stream: MediaStream
 ): [number, number] => {
+  const latestAudioTrackCount = stream ? stream.getAudioTracks().length : 0;
+  const latestVideoTrackCount = stream ? stream.getVideoTracks().length : 0;
+
   const [audioTrackCount, setAudioTrackCount] = useState(
-    stream.getAudioTracks().length
+    stream ? stream.getAudioTracks().length : 0
   );
   const [videoTrackCount, setVideoTrackCount] = useState(
-    stream.getVideoTracks().length
+    stream ? stream.getVideoTracks().length : 0
   );
 
   const tracksChanged = useCallback(() => {
-    setAudioTrackCount(stream.getAudioTracks().length);
-    setVideoTrackCount(stream.getVideoTracks().length);
+    setAudioTrackCount(stream ? stream.getAudioTracks().length : 0);
+    setVideoTrackCount(stream ? stream.getVideoTracks().length : 0);
   }, [stream]);
 
   useEventTarget(stream, "addtrack", tracksChanged);
   useEventTarget(stream, "removetrack", tracksChanged);
 
+  if (
+    latestAudioTrackCount !== audioTrackCount ||
+    latestVideoTrackCount !== videoTrackCount
+  ) {
+    tracksChanged();
+  }
+
   return [audioTrackCount, videoTrackCount];
 };