add is_screencast to VideoSource

[theomonnom]

When is_screencast() == true, WebRTC:

1. Maintains resolution, drops frames instead — uses MAINTAIN_RESOLUTION degradation preference, so under congestion it reduces framerate rather than downscaling (keeps text crisp)
2. Disables quality scaling — won't automatically resize the video to save bandwidth
3. Disables denoising — avoids blurring that damages text/UI readability
4. Guarantees minimum 1200 kbps bitrate floor
5. Enables zero hertz mode — stops sending frames when the screen is static (saves bandwidth)
6. Sets content type to kScreen — which changes encoder config throughout the pipeline (VP9 inter-layer prediction, simulcast layer allocation, etc.)

[davidzhao]

lg

[davidzhao]

should this be a mode param for future proofing? if there are other presets that we'd want to define. maybe:

• camera
• avatar
• screencast

[theomonnom]

it's closely tied to libwebrtc, so it's very unlikely.

But maybe this should be an option inside our TrackPublishOptions API

[ladvoc]

LGTM ✅

[theomonnom]

ahaha this timing