Log Message:
graphics/R-polyclip: import R-polyclip-1.10.0

R port of Angus Johnson's open source library Clipper. Performs
polygon clipping operations (intersection, union, set minus, set
difference) for polygonal regions of arbitrary complexity, including
holes. Computes offset polygons (spatial buffer zones, morphological
dilations, Minkowski dilations) for polygonal regions and polygonal
lines. Computes Minkowski Sum of general polygons. There is a function
for removing self-intersections from polygon data.