Implement polygon/polyline simplification and smoothing

[Xrayez]

Adds RDP method for polyline simplification:

simplification.mp4

This is useful in case you have a complex shape that you need to simplify for the purposes of increased rendering and/or physics performance.

Adds Chaikin's polyline and polygon smoothing:

smoothing.mp4

This can be useful for, say, procedurally generating collision objects from simpler outlines, for instance creating asteroids.

This is approximate by definition, but if someone would also need to implement interpolated version (just smooth_polyline/polygon), this would make sense to add added in #77.

[Xrayez]

Snippet in case tests need to be updated in the future.

extends Node2D

export var is_polygon = false
var points = PoolVector2Array()


func _input(event):
	if event is InputEventMouseButton and event.pressed and event.button_index == BUTTON_LEFT:
		var pos = get_global_mouse_position()
		pos.x = int(pos.x)
		pos.y = int(pos.y)
		points.push_back(pos)
		update()
	elif event.is_action_pressed("ui_accept"):
		if is_polygon:
			points = GoostGeometry2D.smooth_polygon_approx(points, 1)
		else:
			points = GoostGeometry2D.smooth_polyline_approx(points, 1)
		update()
	elif event.is_action_pressed("ui_cancel"):
		points = GoostGeometry2D.simplify_polyline(points, 100.0)
		update()

func _draw():
	if not is_polygon:
		if points.size() >= 2:
			draw_polyline(points, Color.orange, 2, true)
	else:
		if points.size() >= 3:
			draw_colored_polygon(points, Color.orange)