Fix forward calculation in PathFollow3D for the position at the end of the curve

[raulsntos]

Fixes #50973

The issue was, when the PathFollow3D has its unit_offset set to 1.0 (or offset to the length of the curve), that is the final position at the end of the curve, the rotation was being reset. This was happening because of these lines:

godot/scene/3d/path_3d.cpp

Lines 115 to 122 in 60eb508

	if (rotation_mode == ROTATION_ORIENTED) {
	Vector3 forward = c->interpolate_baked(o_next, cubic) - pos;
	if (forward.length_squared() < CMP_EPSILON2) {
	forward = Vector3(0, 0, 1);
	} else {
	forward.normalize();
	}

When we are the end of the curve, the interpolate_baked method returns the same Vector3 for offset that it does for o_next so when the forward vector is calculated in line 116 the result is Vector3(0, 0, 0) which is not valid so in line 119 it is set to Vector3(0, 0, 1) which is the global forward direction and this produces the effect of having the rotation reset.

To prevent this, this PR also interpolates a previous vector o_prev so if we are at the end of the curve, we can calculate the forward vector with the previous position, in pseudo-code:

if o_next - offset != 0:
	forward = o_next - offset
else:
	forward = offset - o_prev

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.

[akien-mga]

If this is cherry-picked for 3.3, #51358 should be included too.

[akien-mga]

Cherry-picked for 3.3.3.