fn signed_half(style: &ShapeStyle, t: f64) -> (f32, f32)Expand description
Half-extents WITH their sign, for the shapes whose extents encode a direction. Line and Arrow run
from (-w, -h) to (+w, +h), so making them absolute (as half_size must, for a layer size) collapses
every line onto the same diagonal whichever way it was actually drawn.