Fix FM-936: RTMP preroll briefly shows image when in SerialElement and rewound. The fix is to call Video.clear() whenever a VideoElement initiates a seek to the end. Because a seek within a SerialElement applies to all children (e.g. seek to the end causes all children to seek to their ends), we can ensure that each child's Video is cleared as soon as the playhead moves past its endpoint, so that the image is not displayed when the playhead returns to that child (through a seek or a rewind).
