Without this change, get_line_decoration_sizes returns an (inline_size,
block_size) pair, where inline_size is parallel to the line being decorated, and
block_size perpendicular. However, these values are generally used as the
dimensions of an axis-aligned bounding box for the line, not as specific
parameters to the rendering process, so it makes sense to arrange them into a
LayoutSize value in this function, since it is already taking the orientation
into account anyway.

The caller, SceneBuilder::add_line, then doesn't need to swap the components,
and the adjustment of the clipping rectangle to avoid partial dots looks a bit
more natural: widths with widths, heights with heights.