This reftest fails with my changes from
https://github.com/servo/webrender/pull/3010. I tried to debug locally but it
passes here.

The reason it fails is because WebRender does distance AA between the segments
of a ridge / groove border, and there's a subpixel difference so we get the AA
slightly different in the test.

Gecko on the other hand represents these borders as different composed solid
segments, so it can't have this problem, since it doesn't do AA across the
segments.

We may want to change that, but it's not clear to me it's wanted, since rounded
corners for these look much better on WR, for example.

Rather than fuzzing the test or something like that, make the test not rely on
that, given it's testing the position of the legend.