Return a line tangent to the polyline at point that lies
length away from the beginning of the polyline.
length is provided, the algorithm starts looking from the end of the polyline. If
length is higher than the length of the polyline, a line tangent to the closest valid polyline endpoint is returned instead.
The tangent line starts at the point at
length. The direction from
end is the same as the direction of the polyline segment at the specified point. If the specified point is a point of discontinuity (e.g. it is a point shared by two polyline segments with different slopes), the tangent line is constructed for the earlier segment (i.e. the segment closer to the beginning of the polyline).
The algorithm ignores polyline segments that have zero length. If the polyline contains no valid segments (i.e. all polyline points are coincident),
null is returned. If the polyline has fewer than two points (exclusive),
null is returned, as well. The
polyline.isDifferentiable() function may be used in advance to determine whether the polyline contains at least one valid segment for wich a tangent may be found.