Function justify_layout 

pub fn justify_layout(
    justify: Justify,
    count: usize,
    base_gap: f32,
    used: f32,
    avail: f32,
) -> (f32, f32)

Computes the leading offset and inter-item gap for a justified line.

Returns (start_offset, added_gap); used is the space already occupied by items and avail is the available main-axis space.