Handle corner cases for curve baking

[xiongyaohua]

When control point and in/out point have equal position, the derivative of bezier curve is 0 vector, which cause error message in Basis::look_at().

ERROR: The target vector can't be zero.
at: looking_at (core/math/basis.cpp:1041)

Also cause #69789

This commit handles this corner case.
Fix #69789

[TokageItLab]

Corner cases should be handled (checked) in the Curve class, not in the Math class.

[xiongyaohua]

To be more precise, they are the corner cases of how to correctly calculate tangent for one bezier segment. And like Vector2D/3D::bezier_derivative(), they can be used more generally than only in Curve2D/3D. So I feel it could also be put on Vector2D/3D instead of Curve2D/3D.

[TokageItLab]

I don't think so. There are several ways to think about the treatment of end/sharp points, and the error caused by the Curve class calculation using look_at() should be handled in the Curve class.

[TokageItLab]

I also think it is not a good thing to insist on the word bezier in naming for function that is not actually a bezier curve. If you want to make it generic for some places, the biggest compromise is to expose a static method called Curve2D/3D::calculate_tangent(p_from, p_to, p_pre, p_post, p_weight) in a similar way to PathFollow3D::correct_posture(). Then, there is no meaning there that the Bezier end/sharp points must be treated in a specific way mathematically, and it makes it clear that Godot's curve implies that the end/sharp points have to be treated in that specific way.

[xiongyaohua]

Thanks for the explanation. I think you are right. I moved these methods to Curve2D/3D as internal methods, to keep them as implementation detail.

[akien-mga]

Thanks!