docs: edl: minor corrections
video: allow hardware decoding only for certain codecs

In particular, this disables mpeg4. There are some files out there that
use GMC, a usually rarely used and ineffective feature, which is not
supported by most hardware decoders. In these cases the hw decoder
outputs garbage, while software decoding works perfectly fine. We can't
really fallback to software decoding in these cases, because we don't
know that something is wrong in the first place. I can't see any
advantages of hw decoding of mpeg4, so it's better to disable it.
vf: cleanup removed filter entry
vf_sub, vf_dlopen: default struct is not needed
vf: move norm_qscale() to the only filter which uses it
video: remove --flip

The --flip option flipped the image upside-down, by trying to use VO
support, or if not available, by inserting a video filter. I'm not sure
why it existed. Maybe it was important in ancient times when VfW based
decoders output an image this way (but even then, flipping an image is a
free operation by negating the stride).

One nice thing about this is that it provided a possible path for
implementing video orientation, which is a feature we should probably
support eventually. The important part is that it would be for free for
VOs that support it, and would work even with hardware decoding.

But for now get rid of it. It's useless, trivial, stands in the way, and
supporting video orientation would require solving other problems first.
video/filter: remove unneeded config callbacks

They didn't do anything.

vf_screenshot.c actually did release the previous image, but that's not
really required. At worst you could take a screenshot and get an old
frame when there's no new frame yet.
vf: declare config() as legacy