Preferr RRID for retransmission over RTX

[nils-ohlmeier]

Description

When receiving an offer with simulcast enabled from Chrome the offer contains RRID and RTX both offered to be used for retransmitting missing packets. Without this PR Pion's answer SDP would contain both mechanisms. That results in Chrome actually sending the retransmission twice, once over RRID and a second time over RTX.

This PR results in Pion preferring RRID and omitting RTX from the m-section payloads to avoid this double retransmission.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 82.75%. Comparing base (7d8a700) to head (e70f6fc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3250      +/-   ##
==========================================
- Coverage   82.87%   82.75%   -0.12%     
==========================================
  Files          80       80              
  Lines       11196    11206      +10     
==========================================
- Hits         9279     9274       -5     
- Misses       1498     1509      +11     
- Partials      419      423       +4     

Flag	Coverage Δ
go	82.75% <100.00%> (-0.12%)	:arrow_down:

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[nils-ohlmeier]

I think to verify only the signaling part should not be too hard. If you want to verify the retransmission behavior on the wire as well, that would get a lot more tricky. I'll start looking into a signaling/SDP test.

[philipch07]

@nils-ohlmeier I think this PR is good to merge if you can add a test unless there's something else that you maybe wanted to update!