Decouple core classes from hardcoded statics.
 - Expose core classes under eburp global object.  eburp.Point, eburp.SceneObject, etc.
 - Remove dependence on hardcoded sprite sheet images.
 - Remove static SceneView.UNIT, and set @unitSize on each SceneView.  Use generously.
 - Move Screen/Util out of core and into game/ so they can be loaded after the core startup.