Clipper 6.4.2 is used internally to perform polypaths clipping, as well
as inflating/deflating polypaths. The following methods were added:
```
Geometry.merge_polygons_2d(poly_a, poly_b) # union
Geometry.clip_polygons_2d(poly_a, poly_b) # difference
Geometry.intersect_polygons_2d(poly_a, poly_b) # intersection
Geometry.exclude_polygons_2d(poly_a, poly_b) # xor
Geometry.clip_polyline_with_polygon_2d(poly_a, poly_b)
Geometry.intersect_polyline_with_polygon_2d(poly_a, poly_b)
Geometry.offset_polygon_2d(polygon, delta) # inflate/deflate
Geometry.offset_polyline_2d(polyline, delta) # returns polygons
// This one helps to implement CSG-like behaviour:
Geometry.transform_points_2d(points, transform)
```
All the methods return an array of polygons/polylines. The resulting
polygons could possibly be holes which could be checked with
`Geometry.is_polygon_clockwise()` which was exposed to scripting as well.
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