This interface allows the ImageFetcher class to drive the production
of image data in an ImageProducer class by calling the doFetch()
method from one of a pool of threads which are created to facilitate
asynchronous delivery of image data outside of the standard system
threads which manage the applications User Interface.
An ImageFetcher is a thread used to fetch ImageFetchable objects.
Once an ImageFetchable object has been fetched, the ImageFetcher
thread may also be used to animate it if necessary, via the
startingAnimation() / stoppingAnimation() methods.
There can be up to FetcherInfo.MAX_NUM_FETCHERS_PER_APPCONTEXT
ImageFetcher threads for each AppContext.