vf_scale: add force_original_aspect_ratio
doc/filters: Reformat scale filter doc

ITU-R BT.470-6 is superseded in 2005. Its successor, BT.470-7, is just a
wrapper to BT.1700 and BT.1701-1. It turns out that 1700 and 1701-1 has some
overlapping parts, but with different names. As a result, I add 1700 and
1701-1, and keep 470-6 because of its popularity.