Added pingpong built-in function

[Chaosus]

This math function used in Unity(https://docs.unity3d.com/ScriptReference/Mathf.PingPong.html), Blender, and (perhaps) many other engines. The proposal is to give users the same capability to use it as in them. This implementation is a copy from Blender with just cosmetical changes(should work as a Unity version, except for negative length values(which are not supported in the Unity for this function anyway)). Added to GDScript / C# / VisualScript.

[Calinou]

What are some practical use cases for this function? It's usually better to use sin() or cos() for smoothly pulsing lights or emissive materials. (Linear pulsing lights or emissive materials often look bad in action.)

While I'm not against this change, I would advise using methods that result in smooth animations whenever possible.

[Chaosus]

@Calinou I have no idea by myself since I'm not using it, maybe some former Unity users may say why they like it and whether they want it in the Godot.

[aaronfranke]

@Chaosus We don't just add features in case maybe someone will use it one day. This needs a proposal. If you aren't using it and don't know how it will be used, that is hardly a substantive use case. So I'm closing this as per the Godot proposal policy (since there is no use case here, one needs to be presented in a proposal before this can be considered).

3. All proposals must be linked to a substantive use-case. In justifying your proposal, it is not enough to say it would be "nice" or "helpful". Use the template to show how Godot is not currently meeting your needs and then explain how your proposal will meet a particular need.

[Chaosus]

Ok, I forgot about it a bit...

[TokageItLab]

Please restore this PR, so I need this for #48332!

That PR is to implement a ping-pong loop in the animation function, and without this function, it will have to write the same mathematical expression again and again.

In short, instead of normal loop using fposmod(), the ping-pong loop needs pingpong().

[fire]

@Chaosus is currently unavailable, but we we discussed that if we are able to find a use, we can use it.

So we can put pingpong into your pr.

[TokageItLab]

Yes, it has been merged into my PR, so there is no need to necessarily restore this one unless a step-by-step merge is required.

[aaronfranke]

For the record, this has been implemented in #53819