Fix simulcast for firefox

[haaspors]

This is a patchset which at least improves using webrtc-rs on a SFU with simulcast and firefox as a client

I found that the simulcast example didn't work for Firefox. It might not really work now either, but this should be a step on the way.
Please let me know what you think :)

[codecov]

Codecov Report

Attention: Patch coverage is 44.82759% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 61.71%. Comparing base (4e09f92) to head (e8d776f).

Files	Patch %	Lines
.../rtp_transceiver/rtp_receiver/rtp_receiver_test.rs	44.44%	0 Missing and 10 partials ⚠️
...tc/src/peer_connection/peer_connection_internal.rs	45.45%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   61.70%   61.71%           
=======================================
  Files         530      530           
  Lines       48848    48847    -1     
  Branches    12274    12268    -6     
=======================================
+ Hits        30142    30146    +4     
- Misses       9542     9545    +3     
+ Partials     9164     9156    -8     

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[haaspors]

Ping @rainliu

Please leave some feedback on what you think about this. Thanks!