closest_point_on_segment_xy returns a 3D point (Z corrdinate !=0)

[sanzoghenzo]

Describe the bug

If the closest_point_on_segment_xy is one of the endpoints, it is returned with z coordinate different than 0.
This is because the logic returns the endpoint as-is, without zeroing the Z coordinate.

To Reproduce

from compas.geometry import Point, Line, closest_point_on_segment_xy


def test_closest_point_segment_xy():
    point = Point(0, 0, -10)
    line = Line([1, 1, -15], [10, 10, 20])
    ponl= closest_point_on_segment_xy(point, line)
    assert ponl == Point(1, 1, 0)

it returns

=============================== FAILURES ================================
_________________ test_closest_point_segment_xy ________________________

    def test_closest_point_segment_xy():
        point = Point(0, 0, -10)
        line = Line([1, 1, -15], [10, 10, 20])
        ponl = closest_point_on_segment_xy(point, line)
>       assert ponl == Point(1, 1, 0)
E       assert Point(1.000, 1.000, -15.000) == Point(1.000, 1.000, 0.000)
E         Use -v to get the full diff