media-poster should go away on play-head move

[mash-graz]

Right now the media-poster only goes away if play is pressed.

This hides those static frame display changes that are shown, when one moves the play head by selecting an arbitrary position in the time-position slider even in paused state as long as no media-poster is specified.

It's just a minor flaw, but it can be annoying, if you search for a scene in a movie without pressing play button at least once first.