Refactor Renderer\Backend\* -> Renderer\

* Create ImageRendererInterface for consumption by decorators.
 * Refactor ImageRenderer to AbstractImageRenderer, fix methods, remove references to Backends, create stub methods for child classes.
 * Remove Renderer\Backend\BackendInterface.
 * Refactor all Renderer Renderer\Backend\* to Renderer\.
 * Rename Decorator\DecoratorInterface to Decorator\ImageDecoratorInterface because it was image-specific and only working with select renderers.