Function soft_shadow_factor 

pub fn soft_shadow_factor(
    hit_pos: [f64; 3],
    hit_normal: [f64; 3],
    light: &AreaLight,
    bvh: &Bvh,
    triangles: &[Triangle],
    num_samples: usize,
    samples: &[[f64; 2]],
) -> f64

Compute soft shadow factor (0=fully shadowed, 1=fully lit) by sampling an area light with num_samples jittered samples.