video_filter/canvas: various aspect-ratio related fixes
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 <88b345e2e0969920adc1eaa21d1a29b223865bcf@videolan.org>

vlc-mirror/vlc-2.1,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.1,shyamalschandra/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.1,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,jomanmuk/vlc-2.2,krichter722/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1