Converts an ImageProvider's settings plus an ImageConfiguration to a key that describes the precise image to load.
The type of the key is determined by the subclass. It is a value that
unambiguously identifies the image (including its scale) that the load
method will fetch. Different ImageProviders given the same constructor
arguments and ImageConfiguration objects should return keys that are
'==' to each other (possibly by using a class for the key that itself
@protected Future<T> obtainKey(ImageConfiguration configuration);