Implement Clipper external library wrapper to GDScript

[TotCac]

I saw that clipper library was added to external libraries. It seems to be used in Sprite for example.
Is it possible to write a wrapper to allow usage directly in GDSript?

It will allow polygon union, intersection, etc.... in 2D

Issue related #6740

Godot version:
3.2

OS/device including version:
All

Issue description:
N/A

Steps to reproduce:
N/A

Minimal reproduction project:
N/A

[Xrayez]

I could work on that, having dealt with Clipper for about one year now for the game I'm making.

See my WIP Clipper module for Godot (well basically working but needs testing):

https://github.com/Xrayez/godot-clipper

Note that it's not 6.4.2 which is present in Godot's source currently. The code is taken from Angus Johnson's new Clipper2 (beta but looks stable), which adds additional features like polygon triangulation along with boolean operations.

I just wonder what are the requirements to make it more Godot-friendly, and possibly other implementation would be more suitable, currently I just combined the three modules into one: polygon clipping, offsetting and triangulating.

[TotCac]

We need @akien-mga or @reduz advice for that i suppose. Perhaps you could join them on irc dev to discuss that.
Sorry I am not a big help ^^