Fix beveling on canvas edges.

[mbostock]

I think we need to special-case the segments that are added on the canvas edges, rather than treating it as if the virtual pixels surrounding the canvas were always excluded from the contour.

[mbostock]

Unfortunately I think the only solution here is to overdraw. Even if you use straight edges rather than bevels, it’s still not correct when smoothing enabled—you need the additional information from outside the displayed grid.

[Fil]

Closing due to inactivity (and no solution, even in theory!)