Function feed_test_pattern 

fn feed_test_pattern(
    encoder: Arc<Mutex<Option<Box<dyn VideoEncoder>>>>,
    cancel: Arc<AtomicBool>,
    width: u32,
    height: u32,
    framerate: u32,
    channels: u8,
    sample_rate: u32,
    palette_seed: u64,
)

Test-pattern feed loop running on a dedicated thread. Pushes a solid colour BGRA frame at the encoder’s framerate (and a chunk of silence at the audio sample rate) until cancel fires.