forcedPicturesOnly

Draw only the pictures an image subtitle track marks as forced (#513), as mpv's sub-forced-events-only does. A Blu-ray or DVD track often holds the full subtitles and a few forced captions together, and this shows a viewer only the captions for the lines in another language and the signs. A track the container flags as forced counts as forced whole, and a text track has no such mark, so both draw as ever. Applies to both subtitle slots, and KitePlayer.setForcedPicturesOnly changes it while playing.