Impl more line, circle.
Impl CutArea.
Move triangle functions into triangle.go
Impl Triangulate.
Impl TranglesIntersectionArea.
Move Triangulate to triangle.go
Impl PolygonIntersectionArea.
Impl LineIntersectionsConvexHull.
Impl RelationCircle*.
Impl HasCommonPointCircleTriangle.
Impl IntersectionPointCircleLine.
Impl IntersectionPointCircleCircle.
Impl CircleCircleIntersectionArea.
Impl CircleTagents.
Impl MinCircleCover.
Impl HalfPlane.