Fix broken root motion scale & Refactor API & Add sample codes in documentation

[TokageItLab]

Fixes #58061

1. Fix root motion scale
   The current root motion scale is completely broken due to incorrect initial values and application process. This PR fix the initial value and the application process, and make the returned root motion scale to be able to used by the accumulator¹.

Before with transform.basis *= root_motion_transform.basis:

root1.mp4

After with accumulator:

root2.mp4

rootmotiondemo.zip

2. Match motion in RootMotionView and AnimationPlayer
   The apply order of the root motion position and root motion rotation differs between AnimationPlayer and RootMotionView. Considering the order in which Bone Pose is applied, the root motion position shuold not affected by the root motion rotation, so the current motion in RootMotionView is incorrect, fixed.

3. Add sample code to document for apply root motion
   In above case, the root motion rotation must respect the object's orientation at the start of the animation. So I added the sample code in the documentation. Also, the scale needs to respect the object's value and accumulator at the start of the animation as well. I added it in the documentation too.

4. Change API get_root_motion_transform() to get_root_motion_position/rotation/scale()
   For these reasons, it makes more sense to separate them as TRS than to take the root motion out as Transform3D.

Footnotes

1. For example, if you animate 4 times and want the Scale to transition like 1 -> 3 -> 5 -> 7, you do not need to use the accumulator, but If you want to do Scale transitions like 1 -> 3 -> 9 -> 27, you need accumulator. ↩

[akien-mga]

Thanks!