obs-webrtc: Add Simulcast Support

[Sean-Der]

Description

This PR adds supports for Simulcast to the obs-webrtc output. Simulcast allows for multiple quality levels to be sent over one track in WebRTC.

RFC 8853 is a technical description. For a less technical explanation Wowza, Dolby and LiveKit have all written articles on it.

This PR starts with adding a checkbox to enable it when the WHIP Service is enabled.

When enabled it sends two additional video streams. They are encoded at 1/2 and 1/4 bitrate + resolution.

Motivation and Context

• Low Latency - Decoding/Re-encoding adds too much latency for interactive content
• Make self hosting easier - Running ffmpeg is costly and difficult to scale
• Streamers more control - If streamers can generate transcodes they control the quality
• E2E Encryption/Prevent Modification- Streamers can distribute keys to viewers and servers can't modify video anymore.

Some questions that have been asked by reviewers already.

What about Enhanced Broadcasting? - Simulcast is part of an existing standard that already sees a billion minutes per day so it is here to stay and is already deployed massively. I would like to see Enhanced Broadcasting in the JSON blob be able to specify the protocol. When a user starts a enhanced broadcasting session it should be able to switch between RTMP/WebRTC/$x. I think both flows can/should exist because they serve different purposes.

UI/UX Considerations - A single opt-in checkbox is non-intrusive and respects the user. The configuration of 50% and 25% is not a standard, but is a common pattern. It would be good to get feedback from all OBS WebRTC users (Dolby, LiveKit, Cloudflare etc...) that this is a sound decision though.

How Has This Been Tested?

This has been tested with the nvenc, qsv, and x264.

I have tested this against Twitch, Broadcast Box, LiveKit and Dolby.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Sean-Der]

Hi @tytan652 I see you gave a 👎 was that an accident? Do you have any feedback/things I can improve?

[tytan652]

Hi @tytan652 I see you gave a 👎 was that an accident?

No, I used that to avoid always adding an "off-topic" to PRs saying that the PR adds code complexity to the streaming codepath which adds more work to my plans for a Service Overhaul.

Do you have any feedback/things I can improve?

No, this is why I just reacted.

[czoli1976]

About time we get Simulcast for WHIP, holy cow.