Make Spine configurable with custom Sprite and Mesh constructors

[bchevalier]

This change can allow the user to make their Spine Sprite and Mesh anything they want.
It was the simplest way I could think of to add illumination to Spine characters.

Here is how the spine animation instantiation changes:

const illuminatedAnimation = new Spine(
        spineSkeletonData,
        SpineIlluminatedSprite,
        SpineIlluminatedMesh,
  );

Where SpineIlluminatedSprite is a custom class that satisfy the SpineSprite property requirements:

class SpineIlluminatedSprite extends IlluminatedSprite {
  region?: TextureRegion = null;
  attachment?: IAttachment = null;
  spineAnimation?: Spine = null;

  constructor(texture: Texture) {
    // some initialization stuff here
  }
}

Problems

1. This requirement is not enforced by the typescript syntax, I wonder what is the best way to achieve this. The main issue is that my custom spine component has to be an extension of both a custom sprite and a spine sprite to work!
   The best solution, in my opinion, would be change the SpineSprite to use composition and include a sprite property instead of inheriting from the Sprite class. This would also reduce the amount of de-opts in the pixi rendering pipeline.
2. This change is made on v3 and not v4 because our project can only use v3 at the moment! What is the right branch on which to submit this change?

[bchevalier]

@ivanpopelyshev I made the change you requested.
About the version, is there a way that this change can make its way to spine v3 with a new release?
I cannot identify a branch on which to submit the PR.

Note that I branched off of this: https://github.com/pixijs/spine/releases/tag/pixi-spine_v3.1.2

[ivanpopelyshev]

Branch is here: https://github.com/pixijs/spine/tree/pixi6 . It can be a pain (we used rush multimodules) but i think i can release it

[bchevalier]

@ivanpopelyshev thanks, I just changed the base. So the PR is now ready on my side.

[miltoncandelero]

Wait, @bchevalier and @ivanpopelyshev why passing a different constructor instead of overloading the functions that are there for that very purpose?

https://github.com/pixijs/spine/blob/master/packages/base/src/SpineBase.ts#L749

why not make something like

export class IlluminatedSpine extends Spine {
    override newContainer() {
        return new IlluminatedContainer();
    }

    override newSprite(tex: Texture) {
        return new IlluminatedSprite(tex);
    }

    override newGraphics() {
        return new IlluminatedGraphics();
    }

    override newMesh(texture: Texture, vertices?: Float32Array, uvs?: Float32Array, indices?: Uint16Array, drawMode?: number) {
        return new IlluminatedMesh(texture, vertices, uvs, indices, drawMode);
    }
}

[bchevalier]

suggested change works, closing the PR