Implement reverse playback and ping-pong loop in AnimationPlayer and NodeAnimation

[TokageItLab]

For this proposal:

• #2664 Add to support ping-pong loop in AnimationPlayer and playing backwards and ping-pong loop in NodeAnimation of AnimationTree

https://www.youtube.com/watch?v=mBOkTA4_HZE

Note:
This PR contains two PRs haven't merged as a prerequisite. One of them has been rejected in the past because its use was unclear, but this PR will require it. If you want to do the merge in stages, I recommend that you start with those PRs merges.

• #41728 Implement reverse playback for animtree
• #46346 Added pingpong built-in function

Note2:
One problem is that if the TimeScale is set to negative, it is not possible to tell when the animation is finished. This can be solved by using the Backward option in NodeAnimation itself.

For example, if you set

[Animation(forward)]->[TimeScale(-1)]->[OneShot]

this won't work as playing through backward animation (but if you want to temporarily reverse the animation, this is correct).

If you set

[Animation(backward)]->[TimeScale(1)]->[OneShot]
or
[Animation(forward)]->[OneShot]->[TimeScale(-1)]

these will work fine.

[fire]

When you are ready for review can you squash the merge commits?

[TokageItLab]

@fire Sure. I left a commit for other PRs to easily see, but I squashed it just now.

[fire]

Review from pouley said we wanted demos of more cases.

[TokageItLab]

Yes, I already understood that as of #41728, as I needed to test it for all track types. I will have a project file for testing later.

[reduz]

My feeling about this PR is that the implementation is very complex for something that is a very corner case. If it could be done in a way that it does not require modifying Animation and it could just be contained in AnimationPlayer and AnimationTree, it could be better.

[reduz]

I see no other way to do this though, so I guess its ok.

[fire]

I'm curious is PINGPONG the more popular spelling compared to PING_PONG? Trying to avoid renames in the future.

[TokageItLab]

Since @Chaosus 's implementation of the math function is pingpong(), it is easy to see when mixed with other enumerated values, I decided to use pingpong(), XXX_PINGPONG, and pingponged.
However, since I am not so particular about it, I can consider using XXX_PING_PONG as the enumerated type and ping_pong() as the math function.

[TokageItLab]

@akien-mga Could you give me a review when you have a time?

[akien-mga]

Thanks!

[reduz]

I sincerely apologize I had to revert this PR. I am refactoring large part of animation playback code (#53689) and this PR gets in the way. Please wait a couple of days until I am done with my changes to the animation system and attempt a PR against the new code.