Enhance screen background possibilities


Today only a single picture is supported. For the future, the vision is to support
  • Single color
  • Background picture
  • Background layers
  • Scrolling: based either on entity position or time; wrapping could also be supported; each background layer can have a speed.
    The background picture is also stretched to fit the screen. There should be a way to support background pictures bigger than the screen dimensions.
    After this support is implemented in ArcadEngine, the GameDefinitionDSL (and eventually others) should be updated to enable users to use the feature.