Set is_screencast on NativeVideoSource creation

[iparaskev]

Makes InternalSource's is_screencast configurable when creating a NativeVideoSource. This improves the stream's latency when vp9 is used for sharing screen content.

[ilo-nanpa]

it seems like you haven't added any nanpa changeset files to this PR.

if this pull request includes changes to code, make sure to add a changeset, by writing a file to .nanpa/<unique-name>.kdl:

minor type="added" "Introduce frobnication algorithm"

refer to the manpage for more information.

[typester]

It looks like we already have a screenshare flag:

https://github.com/iparaskev/rust-sdks/blob/d276130dbd0205520f272861da718ce84b4bef80/livekit/src/room/options.rs#L76-L88

#[derive(Clone, Debug)]
pub struct TrackPublishOptions {
    // If the encodings aren't set, LiveKit will compute the most appropriate ones
    pub video_encoding: Option<VideoEncoding>,
    pub audio_encoding: Option<AudioEncoding>,
    pub video_codec: VideoCodec,
    pub dtx: bool,
    pub red: bool,
    pub simulcast: bool,
    // pub name: String,
    pub source: TrackSource,
    pub stream: String,
}

https://github.com/iparaskev/rust-sdks/blob/d276130dbd0205520f272861da718ce84b4bef80/livekit/src/room/track/mod.rs#L61-L68

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum TrackSource {
    Unknown,
    Camera,
    Microphone,
    Screenshare,
    ScreenshareAudio,
}

Would it be sufficient to reuse this flag?

cc: @theomonnom

[iparaskev]

Would it be sufficient to reuse this flag?

I didn't think of this TBH. It could be done by exposing a setter in VideoTrackSource and then set the is_screencast_ member in publish_track. If you are OK with this, I am happy to go with it.

Side note: The only "problem" I see with this, is that NativeVideoSource will require a kind of two stage initialization, which could be error prone. A way to mitigate it would be to create the NativeVideoSource object in publish_track, but I am not sure if this is acceptable.

[iparaskev]

Something that I had forgotten to mention when I created this, is that when is_screencast is set, libwebrtc limits the FPS to 5. I had to add spatial_layers.back().maxFramerate = video_codec.maxFramerate; here as a workaround.