wall.c: set the total output aspect ratio to the device aspect ratio, even

wall.c: set the total output aspect ratio to the device aspect ratio, even
if size constrains stoped us from making the parts have that aspect ratio.
This causes a slight stretch of the image in some direction but should
totally remove black lines between the elements.
todo:
 * make device aspect ratio configureable (easy)
 * handle anamorphic input video (medium)