palette, implemented the "force display" command as forever-living
    subtitles, and reworked some code.
  * ./src/video_output/video_output.c: we increment the image date even if
    we are repeating the previous image, so that subtitles have a chance to
    get displayed.
  * ./src/video_output/vout_subpictures.c: ephemer subpictures don't timeout.