Investigate line antialiasing

[rastertail]

Right now I'm pretty sure the antialiasing algorithm used for line rendering is just an approximation; it would be nice to compute the exact value based on the trapezoidal coverage of the pixel. I'm almost certain it's possible, but i looked at it a while ago and wasn't able to work through the math. I doubt there would really be much of a noticeable difference, so this would probably be an opt-in feature.