A policy for discarding tile images that contain no data (and so aren't actually images).
This policy discards 
    
    DiscardEmptyTileImagePolicy.EMPTY_IMAGE, which is
expected to be used in place of any empty tile images by the image loading code.
Members
    Default value for representing an empty image.
Methods
    Determines if the discard policy is ready to process images.
Returns:
    True if the discard policy is ready to process images; otherwise, false.
    
    Given a tile image, decide whether to discard that image.
    
| Name | Type | Description | 
|---|---|---|
| image | HTMLImageElement | An image to test. | 
Returns:
    True if the image should be discarded; otherwise, false.
    
