Fix: allow render function in RenderContainer to be overridable

[GoodBoyDigital]

allow devs to either provide a render function or override it by extending the class:

import { RenderContainer } from 'pixi.js';

// extend the class
class MyRenderContainer extends RenderContainer
{
   render(renderer)
   {
     renderer.clear({
        clearColor: 'green', // clear the screen to green when rendering this item
     });
  }
}

// override the render method
const renderContainer = new RenderContainer(
(renderer) =>  {
    renderer.clear({
      clearColor: 'green', // clear the screen to green when rendering this item
    });
});

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