This forces us to diverge from the spec, but it's for a good cause:
by moving it into ImageRequest, we'll be able to reuse fetching and
decoding logic from CSS and other places.
This patch also makes ImageRequests shareable, currently keyed by
the URL (this part needs improvement!)
This patch adds HTML::ImageRequest and HTML::DecodedImageData.
The latter had to use a different name than "ImageData", as there is
already an IDL-exposed ImageData class in HTML.