Function session_fullcolor 

pub fn session_fullcolor(
    encoder: Option<&FrameEncoder>,
    settings: &RustCaptureSettings,
) -> bool

The chroma format a session actually carries, which is not always the one requested: a hardware session only when the device carries it, the software path only when the build’s encoder for the codec does. Every consumer of “is this stream 4:4:4” reads it from here.