Add root motion accumulator to fix broken RootMotionView

[TokageItLab]

Fixes #72930.

rootmotionfix.mp4

Now we can get the actual blended rotation of the rotation with get_root_motion_rotation_accumulator(), not just the delta value.

The following would allow the root motion position to be correctly applied while taking the object's orientation into account, without keeping the current rotation at the start of the animation. Now we can close #58061 completely.

func _process(delta):
	if Input.is_action_just_pressed("animate"):
		state_machine.travel("Animate")
	set_quaternion(get_quaternion() * animation_tree.get_root_motion_rotation())
	var velocity: Vector3 = (animation_tree.get_root_motion_rotation_accumulator().inverse() * get_quaternion()) * animation_tree.get_root_motion_position() / delta
	set_velocity(velocity)
	move_and_slide()

I added a explanation of this code to the documentation. This should work even if the animation is Xfade or interrupted.

Also, using get_root_motion_rotation_accumulator() should be able to solve issue #67125. Closes #67125.

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We should discuss whether this is the good naming for the methods.

I think get_root_motion_rotation_delta() and get_root_motion_rotation() would be better, but it would be breaks compat; This means to rename the current get_root_motion_rotation() to get_root_motion_rotation_delta() and to rename this PR's get_root_motion_rotation_accumulator() to get_root_motion_rotation().

Probably it is a better naming like get_root_motion_rotation_base() or get_root_motion_rotation_source()?

What do you think? @reduz @fire @lyuma @SaracenOne

[TokageItLab]

The explanation in the document was incorrect and has been corrected.

[SaracenOne]

I think this makes sense to me 👍

[akien-mga]

Thanks!