Function first_los_block 

pub fn first_los_block(
    from_x: f32,
    from_y: f32,
    from_z: f32,
    to_x: f32,
    to_y: f32,
    to_z: f32,
    obstacles: &LosObstacles,
    ground_z: impl Fn(f32, f32) -> f32,
) -> Option<LosBlock>

First occlusion along the 3D ray, if any.