Anti-aliasing for straight boundary lines

Straight lines on the outline are currently not anti-aliased. For horizontal/vertical lines, this will just look like they snap to pixel border (so fidelity to original shape suffers), and for diagonal lines it will look aliased.

Maybe it is possible to use some derivatives trick for these lines as well? Or maybe something like what we do in the rectangle shader, although I think that may only be for axis-aligned edges.