Function taffy::compute::grid::track_sizing::cmp_by_cross_flex_then_span_then_start

pub(super) fn cmp_by_cross_flex_then_span_then_start(
    axis: AbstractAxis,
) -> impl FnMut(&GridItem, &GridItem) -> Ordering

To make track sizing efficient we want to order tracks Here a placement is either a Line representing a row-start/row-end or a column-start/column-end