Fix segment intersection consistency in Geometry2D

[pouleyKetchoupp]

Segment collision results could be different depending on the direction when they exactly touch (order of the points in segments). This was due to the way parallelism was checked, using different logic based on positive or negative sign of cross product.

Now the results are the same whatever the direction, without changing the current design, which is that parallel or colinear segments are not considered colinear.
It's still probably safer to make this change only on master to avoid unexpected side effects.

Fixes inconsistencies with raycasts exactly on edges of convex shapes depending on the direction.
Fixes #52088
Edit: fixes #41011 as well

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Apart from the physics issue, here's an example that shows inconsistencies.

Script (just needs a Control node to run):

extends Control

var A = Vector2(0, 0)
var B = Vector2(0, 50)
var C = Vector2(50, 0)

var offset = Vector2.ZERO

var font : Font = get_theme_font("Label")

func _draw():
	test_segment(A, B, A, C)
	test_segment(B, A, A, C)

func test_segment(from_a, to_a, from_b, to_b):
	var text = "%s%s / %s%s" % [from_a, to_a, from_b, to_b]
	draw_string(font, offset + Vector2(0, -20), text)
	
	from_a = from_a + offset
	to_a = to_a + offset
	from_b = from_b + offset
	to_b = to_b + offset
	
	var result = Geometry2D.segment_intersects_segment(from_a, to_a, from_b, to_b)
	if result:
		draw_circle(result, 5, Color.YELLOW)
	
	draw_arrow(from_a, to_a, Color.GREEN)
	draw_arrow(from_b, to_b, Color.BLUE)
	
	offset = offset + Vector2(0, 100)

func draw_arrow(from, to, color):
	draw_line(from, to, color)
	
	var dir = (to - from).normalized()
	var tangent = dir.orthogonal()
	
	draw_line(to, to + tangent * 3 - dir * 5, color)
	draw_line(to, to - tangent * 3 - dir * 5, color)

Result (segments in green and blue, shows a yellow dot when there is an intersection):

With this PR, both of these cases detect the intersection the same way:

[pouleyKetchoupp]

Checks are currently failing on this unit test:

 tests/test_geometry_2d.h:143: ERROR: CHECK_FALSE( Geometry2D::segment_intersects_segment(Vector2(-1, 1), Vector2(1, -1), Vector2(0, 1), Vector2(1, -1), &r) ) is NOT correct!
  values: CHECK_FALSE( true )
  logged: Parallel segments should not intersect.

It says Parallel segments should not intersect but the segments used in the test are not parallel so I'm not sure what the intention was.
@mbrlabs Do you remember if this test is covering a specific use case the engine relies on? I see some discussions in #44274 about is_point_in_* functions, but not about the segment/segment intersection.
Otherwise I'll probably just change this test and add some new ones to cover the cases this PR is fixing.

[mbrlabs]

That must have been a mistake of mine...at least the error message as they are cleary not parallel and touch in (1, -1). Maybe i tried to test that touching != intersecting at the time but i don't remember.

You can replace the points in the test with Vector2(-1, 1), Vector2(1, -1), Vector2(0, 1), Vector2(2, -1). These are parallel :)

[pouleyKetchoupp]

@mbrlabs Thanks! I've updated the parallel segment test following your suggestion and added new ones for touching segments.

Edit: Also as a note, is_point_in_polygon now returns true on the edge of the polygon, which makes it consistent with is_point_in_circle.

[Xrayez]

Could this fix #41011 as well?

It's still probably safer to make this change only on master to avoid unexpected side effects.

True.

[pouleyKetchoupp]

Yes, it will fix #41011 as well. Thanks! I've added it to the description.