imgILoader in Firefox 3.5

 Start the load and decode of an image.
 @param aURI the URI to load
 @param aInitialDocumentURI the URI that 'initiated' the load -- used for 3rd party cookie blocking
 @param aReferrerURI the 'referring' URI
 @param aLoadGroup Loadgroup to put the image load into
 @param aObserver the observer
 @param aCX some random data
 @param aLoadFlags Load flags for the request
 @param aCacheKey cache key to use for a load if the original
                  image came from a request that had post data
 @param aRequest A newly created, unused imgIRequest object or NULL for one to
                     be created for you.


 libpr0n does NOT keep a strong ref to the observer; this prevents
 reference cycles.  This means that callers of loadImage should
 make sure to Cancel() the resulting request before the observer
 goes away.

 Start the load and decode of an image.
 @param aChannel the channel to load the image from.  This must
                 already be opened before ths method is called, and there
                 must have been no OnDataAvailable calls for it yet.   
 @param aObserver the observer
 @param cx some random data
 @param aListener [out]
        A listener that should receive the data. Can be null, in which
        case imagelib has found a cached image and is not interested in
        the data. The caller needs not cancel the channel in this case.

 libpr0n does NOT keep a strong ref to the observer; this prevents
 reference cycles.  This means that callers of loadImageWithChannel should
 make sure to Cancel() the resulting request before the observer goes away.

 Checks if a decoder for the an image with the given mime type is available
 @param mimeType The type to find a decoder for
 @return true if a decoder is available, false otherwise