Fix loop when an SDP offer is received without video

[giavac]

When an SDP offer is received without video, the current code still performs a getUserMedia() request with video set to true.

Then the local streams are added to the RTCPeerConnection, before the remote description has been added to it.

This triggers an additional onnegotiationneeded that calls startNegotiating() again, which in turns calls _createAnswer() again, which will trigger new onnegotiationneeded events in a loop that continues for the duration of a session.

I'm not sure about the side effect of this loop, but I'm concerned it may appear as missing ICE candidates in the answer SDP, as if the loop is impacting the correct addition of ICE candidates to the answer SDP that's being built.

To reproduce this and see the loop:

1. Add a log line inside onnegotiationneeded
2. Send an audio-only offer to the running client

With the changes in this PR instead the constraint for getUserMedia will depend on the combination of available devices and what was received in the offer. An audio-only offer will only cause a getUserMedia with video false.

[jpsantosbh]

I like this solution better than what I did on the V3.
I'm going to port it over to v3 as well.

[iAmmar7]

LGTM, thanks! 🚀 please rebase your branch with the main.

I might be wrong but just thinking out loud; this seems like a design change considering we will no longer negotiate the video/audio in case the offer SDP does not include the video/audio. This might impact the end user, right? and we probably want to release this as a new minor version of the SDK.

[giavac]

this seems like a design change

Even without these changes the answer SDP from the SDK would not have a video section, if the offer didn't.
What changes is that the local video stream is not created at all.
This would be only visible to the client if getUserMedia triggered a permission request for video, but since video won't be used it is more correct not to try to get those permissions.

Apart from the possibility of regressions that can always happen with changes, it seems to me more like a refactoring or fix, rather than a behaviour change. Let me know what you think.

[iAmmar7]

Even without these changes the answer SDP from the SDK would not have a video section, if the offer didn't.

Gotcha, then I think it's fine. Thanks for the details. 👍

[iAmmar7]

🥇