AnimationTree TimeScale with negative value fails with looping animation

[guzzard]

Godot version:
3.1 stable mono

OS/device including version:
Arch Linux fully updated

Issue description:
When setting negative value in time scale node animation begin playing backwards, but it does not loop, it just rewinds to the beginning and stops there. Playing forward the animation loops as expected.

Steps to reproduce:

1. Setup animation tree with time scale node.
2. Set time scale to negative value through script.

Minimal reproduction project:
TimeScaleTest.zip
On start the animation will play forwards automatically. Press 'left' to play backwards, and watch the animation get stuck after rewinding.

[Banderi]

I've encountered this issue too when playing with the proper 'play_backwards()' method (or setting the "from_end" flag to "true")
The animation correctly plays backwards, but if the animation controller is set to loop or the animation's set to follow into itself (via .animation_set_next()) the animation will retain the speed assigned, but only will play backwards for one cycle.

I assume internally it's just set to call .play() whenever it reaches the end, without remembering the additional flags?
I'm working on 3.2 so it might have changed from OP's version - but if I set a negative speed it just freezes in place, which makes me think it's the same issue at the core: it's trying to play another cycle, but it forgot to do it backwards.

[epth]

I have tried in 3.2.2-stable, it still fail.

[Calinou]

Duplicate of #21665.