Option to constrain arcs to have continuous tangents

[PureAsbestos]

Feature Description

A common need when editing paths is to have them smoothly connect together.

For arcs this looks like changing the radius until the tangents align:

It would be very convenient to be able to do this automatically and with good precision. Not sure if it is possible to do this with different rx and ry, but I only need it for circular arcs. Related to #1373

[MewPurPur]

I definitely want to have this in GodSVG, sooner or later.

[PureAsbestos]

given endpoints a and b, and given the tangent at a, it seems we can get the radius like this:

func find_radius_from_tangent(a : Vector2, b : Vector2, tangent : Vector2) -> float:
	var perpendicular_dir = tangent.orthogonal()
	var midpoint = (a+b)/2.0
	var bisector_dir = (b-a).orthogonal()
	var circle_center = Geometry2D.line_intersects_line(a, perpendicular_dir, midpoint, bisector_dir)
	if circle_center == null:
		# lines are parallel, this happens when we should draw a line segment
		return 0.0
	else:
		return a.distance_to(circle_center)

which corresponds to this geometric picture:

we should also set the large arc and sweep flags based on the inputs:

var r = find_radius_from_tangent(a, b, tangent)
var large_arc = 0
if (b-a).dot(tangent) < 0.0:
	large_arc = 1
var sweep = 0
if (b-a).orthogonal().dot(tangent) > 0.0:
	sweep = 1

[MewPurPur]

Thanks! I don't know when I'll get to it, but this will certainly be useful.