Handle protocol independent urls

subreddit css may contain protocol independent urls like `//a.thumbs.redditmedia.com/_6cQow5wLcUnICddDUm4btvZLuQOEsAiKwtazmQ9m_0.png`, but a schema is needed to fetch this urls. This tweaks the apng workaround code to match http:// https:// and // at the beginning of an url and replace it with the s3 url.