This modification will only keep incoming tracks from media sections
with `a=sendrecv` or `a=sendonly` attributes. This modification is not
necessary for pion-to-pion peer connections, but for browser-to-pion
connections using Unified Plan.
Background:
When a `RTCPeerConnection#removeTrack()` is called in the browser and
the connection is renegotiated, the browser (tested in Firefox 75.0)
will change the media section attribute to `a=recvonly` from
`a=sendrecv`, or `a=inactive` from `a=sendonly`, but will keep the same
`a=ssrc:<ssrc>` associated with the same media (mid) section.
Previously, when a remote track obtained with Pion's `OnTrack()` handler
was being read from, and this remote track was later removed in the
browser, the track's `Read()` method would never return `io.EOF` (unless
the peer connection was closed).
With this commit, the `trackDetailsFromSDP` function will only include
remote track details whose media section has `a=sendonly` or
`a=sendrecv` attributes, and previously existing logic will ensure that
a receiver for that specific ssrc is stopped, so that
`webrtc.Track#Read()` method returns `io.EOF`.
If the remote description contains rtx information via a `ssrc-group`,
the extra `ssrc` entries must be ignored to ensure thats no receiver
is added for them.
This can solve issues like `Incoming unhandled RTP ssrc(1858531374)` if
those are caused by a wrong receiver getting started for the rtx ssrc
but not for the track ssrc, based on the ssrc/transceiver oder in the
remote description.
Until rtx is properly supported, this change fixes the imediate issue
that some remote streams are randomly not received because of a remotes
rtx information.
Fixes: #1081
Related: #1083
