Function render_direct 

pub fn render_direct(
    config: &RenderConfig,
    camera: &Camera,
    bvh: &Bvh,
    triangles: &[Triangle],
    materials: &[Material],
    lights: &[PointLight],
) -> Vec<[f64; 3]>

Ray trace a simple scene (direct illumination only) on the CPU.

Returns a flat buffer of (r, g, b) per pixel in row-major order.