Enable re-using a video element after "emptied"

[martinstark]

Reset state to initial state to allow a filter to be re used for consecutive plays.

[martinstark]

Unsure if this is a good idea, it should be possible to re-use an old video element by simply creating a new media-event-filter instance with the old video element after it has emptied. It should achieve the same as introducing state-reset functionality to the filter, without adding complexity to the code or the API.