vf_screenshot: remove unnecessary code
video: remove VFCAP_POSTPROC

This wasn't used anymore, not even until recently.
vf_lavfi: export a wrapper function

This will allow old filter to run libavfilter instead by calling
vf_lw_set_graph(), which turns the filter into a wrapper, using a given
libavfilter graph.

Later commits use that to automatically "reroute" a bunch of filters to
libavfilter. We want to get rid of the old MPlayer filter code, because
it's bad an unmaintained, but we still don't want to force everyone to
use vf_lavfi, so this solution will do for a while.
vf_yadif: change options, reroute to vf_lavfi

Also remove the ability to disable deinterlacing at runtime. You can
still disable deinterlacing at runtime by using the ``D`` key and its
automatical filter insertion/removal.
vf_delogo: reroute to vf_lavfi

The ``file`` suboption is unsupported on lavfi.
vf_gradfun: reroute to vf_lavfi

Also update the manpage. We changed defaults and added a suboption quite
some time ago, and we forgot to update the manpage at all.
vf_hqdn3d: change options, reroute to vf_lavfi
vf_stereo3d: reroute to vf_lavfi
vf_noise: reroute to vf_lavfi

Unfortunately, this forces filtering both luma and chroma, because
otherwise we'd have to deal with libavfilter's vf_noise weird handling
of YUV vs. RGB formats. Would we e.g. filter luma only, it would filter
red in RGB mode only, because it goes by component and there's no way to
distinguish YUV and RGB by just using the filter's options.
vf_phase: change options, reroute to vf_lavfi

The option change is probably backwards compatible.
vf_unsharp: change options, reroute to vf_lavfi
vf_pullup: change options, reroute to vf_lavfi

The options are probably mostly backwards compatible.
vf: remove unneeded field

This was once required for padding, because many filters didn't use
designated initializers, and initialized a removed field with NULL.
video/filter: remove vf_down3dright

Apparently you can get this with: stereo3d=ab[2]{l,r}:sbs[2]{l,r}

So it seems the filter is redundant and can be removed.

Also see FFmpeg commit 2f11aa141a01.
vf_divtc: use option parser
vf_dsize: use option parser

Mostly backwards compatible, we don't change much because we just want
to get rid of the legacy option string handling.

You can't pass an aspect as first argument anymore.
vf_eq: use option parser
vf_ilpack: use option parser
vf_pp: use option parser
vf_rotate: use option parser