rotateExtrude doesn't produce solid objects when translating

[isovector]

When translating upwards during a rotateExtrude, like in the following example

main :: IO ()
main = writeSTL 0.5 "/tmp/inrads.stl" $
  rotateExtrude
    180
    Nothing
    (Left (0, 5))  -- go up 5 units
    (Left 0) $
      translate (10, 0) $ rectR 0 0 3

the resulting mesh is missing some of its exterior triangles. Here's the result, rotated to look at the bottom:

[isovector]

I saw something similar to this when I messed up my BB caluclations in #300. I suspect that's the case here. cf ca92130

[isovector]

The fix here is to patch

https://github.com/colah/ImplicitCAD/blob/aea51dd6b61473d76e0ef4020660b63bbbcc30ca/Graphics/Implicit/ObjectUtil/GetBox3.hs#L115

to instead say:

        (V3 (-r) (-r) $ (y1 - abs yshift), V3 r r $ y2 + abs yshift)

[isovector]

I learned today that the behavior of this function is completely different if you use Just 0 instead of Nothing for what I assumed (probably incorrectly) was the rounding parameter.