implementation of the ability to change the base color

[KevinRohn]

I've implemented the ability to change the base color of the model.

There is a ModelViewerColorController which can be used to call the JavaScript function to change the base color of the model.
Currently it only supports the auto detected base color. As I understand the documentation, the auto detection selects the largest area of the model. So if there is just a simple model, you can change the color of the whole object. Otherwise the largest object will just be changed.

Example usage:

// ....
ModelViewerColorController _colorController;

// ....
@override
void initState() {
    super.initState();
    _colorController = ModelViewerColorController();
}

// ....
Future void changeColor() async {
    await _colorController.changeColor("0,1,1,1");
}

// ....
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text("Model Viewer")),
        body: ModelViewer(
          src: 'https://modelviewer.dev/shared-assets/models/Astronaut.glb',
          enableColorChange: true,
          colorController: _colorController,
          alt: "A 3D model of an astronaut",
          ar: true,
          autoRotate: true,
          cameraControls: true,
        ),
      ),
    );
  }
}