Dark line between diagrams at certain widths

[JakeBr]

import Diagrams.Prelude
import Diagrams.Backend.Cairo.CmdLine

main :: IO ()
main = defaultMain dia

dia :: Diagram B R2
dia = (s1 ||| s1) <> s2
  where s1 = square 1 # lw 0 # fc white
        s2 = square 2 # lw 0 # fc crimson

Looks fine with width 100:

But there is a dark line between the squares with width 99:

Tested on Ubuntu 13.04, Cairo 1.12.14

[JakeBr]

Oh, I just noticed that the exact same thing happens with the SVG backend as well.

[fryguybob]

It could be that this is just an artifact of filling areas that do not align with pixels. Some backends might be able to support aligning to the nearest pixel for some drawing actions. It could also be that if we were more careful about some rounding we would avoid it, but I doubt that that would always be able to help. I don't know of a good way to express aligning to pixels :(.