Stretch widescreen mode if difference is negligible

Widescreen mode "zooms" the player's view. Since the player's view (at
640×336) isn't exactly 16:10, and since the changes in commits
2fd72d43596449faf0fc9cc26a3ea12c01233604 and
02a5150055bbadaede4dea49c60f7ff5b717a351, a 16:10 display resulted in
pillarboxes of 7 pixels either side.