Paint factory for paint instances used through out the engine.

# Problem to solve
There is no way to globally customize the default paints used though out the engine. I can't for example, change the default white color paint used on Sprites to a different one without having to explicit pass a custom paint to all the instances.

# Proposal
We could provide global factories, which allows the user to specify their own factories, so the paints could be globally customized.

Example:

```dart
PaintFactories.sprite = () => Paint()..color = Colors.white;
PaintFactories.debugShapes = () => Paint()..color = Colors.magenta;
// ..etc
```

The user could then modify those attributes with new closures to customize to the behaviour they want.