parentMainAxisPosition into the child's coordinate system.
parentMainAxisPosition is a distance from the top edge (for vertical
viewports) or left edge (for horizontal viewports) of the viewport bounds.
This describes a line, perpendicular to the viewport's main axis, heretofor
known as the target line.
This method returns the distance from the leading edge of the given child to the target line described above.
parentMainAxisPosition is not from the leading edge of the
viewport, it's always the top or left edge.)
@protected double computeChildMainAxisPosition(RenderSliver child, double parentMainAxisPosition);