842, 847 slightly enlarge a clip polygon to counteract integer truncation
842, 847 slightly enlarge a clip polygon to counteract integer truncation

Geomery in referenced issues triggered Clipper problems, but also
pointed to a situation where integer truncation (as coordinates pass
into Clipper) might be shrinking a clip polygon in a way that leaves
degenerate or unwanted thin clip results. Growing the clip polygon by 2
is expected to overcome any issues caused by truncation of floats.

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