[BUG] Simulcast can't work in examples/echotest-jsonrpc/index.html

[r1a2y3]

Your environment.

• Version: 7b6cc7b
• Client: 7b6cc7b
• Environement: Linux
• Are you using a TURN server? No

What did you do?

1. git clone https://github.com/pion/ion-sfu.git
2. follow https://github.com/pion/ion-sfu#sfu-with-json-rpc-signaling
   go build ./cmd/signal/json-rpc/main.go && ./main -c config.toml
3. edit signal address to my server ./examples/echotest-jsonrpc/index.html
4. open index.html with browser
5. start with simulcast

What did you expect?

after step 5, video send and receive normally

What happened?

video receives abnormally. it seem's that video can't be correctlly send from ion-sfu in the receiver side.

I tried to understand but failed to get the root cause. It may lie at WebRTCReceiver and DownTrack when switch layer.

Could some guys kindly to help finding the root cause?
THANKS A LOT!