Smoothstep is not a very good approximation because it has zero
derivatives at 0.0 and 1.0. It makes edges too sharp (i.e. more
aliased).

From the comments in `prim_shared.glsl`:

This cubic polynomial approximates the area of a 1x1 pixel square under a
line, given the signed Euclidean distance from the center of the square to
that line. Calculating the *exact* area would require taking into account
not only this distance but also the angle of the line. However, in
practice, this complexity is not required, as the area is roughly the same
regardless of the angle.

The coefficients of this polynomial were determined through least-squares
regression and are accurate to within 2.16% of the total area of the pixel
square 95% of the time, with a maximum error of 3.53%.