video_filter/canvas: various aspect-ratio related fixes

video_filter/canvas: various aspect-ratio related fixes

 - Remove silly defaulting to 4:3.  Instead, compute PAR based
   upon SAR of input video and canvas size.
 - Update documentation to remove ambiguity over what is meant
   by aspect-ratio
 - Fix subpicture calculations to
    (a) be much simpler,
    (b) correctly calculate width and height
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>