The media element must be attached to the DOM.

Having the media element returned from render meant that when we
changed (as a result of a setSources call) to a different media
element we no longer had the player we were using on the attached
DOM.

Using an anyonymous div as our output element, and attaching the
media elements to that as they are created (and removed when
destroyed) allows us to have the single element returned from
render while letting us chop and change the media element within it.