C#: Fix Vector3 Slerp normalization error

[Z0rb14n]

Closes #94049 by adding a check of the rotation axis being the zero vector, and linearly interpolating if so.
The check already exists in the C++ implementation of Slerp, so this PR ports it to C#:

godot/core/math/vector3.h

Lines 238 to 257 in e343dbb

	Vector3 Vector3::slerp(const Vector3 &p_to, real_t p_weight) const {
	// This method seems more complicated than it really is, since we write out
	// the internals of some methods for efficiency (mainly, checking length).
	real_t start_length_sq = length_squared();
	real_t end_length_sq = p_to.length_squared();
	if (unlikely(start_length_sq == 0.0f || end_length_sq == 0.0f)) {
	// Zero length vectors have no angle, so the best we can do is either lerp or throw an error.
	return lerp(p_to, p_weight);
	}
	Vector3 axis = cross(p_to);
	real_t axis_length_sq = axis.length_squared();
	if (unlikely(axis_length_sq == 0.0f)) {
	// Colinear vectors have no rotation axis or angle between them, so the best we can do is lerp.
	return lerp(p_to, p_weight);
	}
	axis /= Math::sqrt(axis_length_sq);
	real_t start_length = Math::sqrt(start_length_sq);
	real_t result_length = Math::lerp(start_length, Math::sqrt(end_length_sq), p_weight);
	real_t angle = angle_to(p_to);
	return rotated(axis, angle * p_weight) * (result_length / start_length);

Sample problematic C# code I used for testing:

public override void _Ready()
{
  Vector3 vecA = new Vector3(0.12f, 0.12f, 0.12f);
  Vector3 vecB = new Vector3(0.512f, 0.512f, 0.512f);
  Vector3 vecC = new Vector3(-0.512f, -0.512f, -0.512f);
  GD.Print(vecA.Slerp(vecB, 0.5f));
  GD.Print(vecA.Slerp(vecC, 0.5f));
  Vector3 v1 = new Vector3(1,2,3);
  Vector3 v2 = new Vector3(2,4,6);
  Vector3 v3 = new Vector3(-3,-6,-9);
  GD.Print(v1.Slerp(v2, 0.5f));
  GD.Print(v1.Slerp(v3, 0.5f));
}

Console output (previously, it would crash on any of the print statements):

(0.316, 0.316, 0.316)
(-0.19600001, -0.19600001, -0.19600001)
(1.5, 3, 4.5)
(-1, -2, -3)

[raulsntos]

Thanks for contributing to the .NET module! The changes look good to me, just a minor nit-pick.

[raulsntos]

Looks good to me, thanks!

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[aaronfranke]

I know it's already merged but I just wanted to comment that I also approve of this ✅

[akien-mga]

Cherry-picked for 4.3.1.