The spec has a complicated algorithm for selecting a <source>
element among multiple <source> children of a HtmlMediaElement
where it loops over all of them, tries each and takes the first
where "everything works out".

This commit implements a much simpler and restricted approach by
just taking the first <source> child, and if that fails,
failing altogether, without looking at any further children.

This is an improvement over the current status and makes gifv
items on imgur playable, although it doesn't mean full <source>
support.