ad_lavc: remove deprecated downmixing by channel count

Downmixing by channel layout now hopefully works with all supported
libavcodec versions.
Remove some more unneeded version checks

All of these check against things that happened before the latest
supported FFmpeg/Libav release.
demux: use av_packet_ref()

av_copy_packet() was FFmpeg specific, av_packet_ref() is now available
on all supported libavcodec releases.
build: simplify libavfilter configure checks

This is all not needed anymore. In particular, remove all configure
switches except --enable-libavfilter.
vf_gradfun: remove internal implementation

This follows the (only slowly progressing) plan to replace all internal
video filters with libavfilter.

All what's left in vf_gradfun.c is the weird wrapper around vf_lavfi.c.
vf_unsharp: remove internal implementation

See previous commit.
vf_yadif: remove internal implementation

See previous commit.
vf_hqdn3d: remove internal implementation

See previous commit.
vf_delogo: remove internal implementation

See previous commit.
ad_lavc: use new AVFrame API

Set refcounted_frames, because in some versions of libavcodec mixing the
new AVFrame API and non-refcounted decoding could cause memory
corruption. Likewise, it's probably still required to unref a frame
before calling the decoder.
image_writer: use new AVFrame API
encode: use new AVFrame API
ao_lavc: set AVFrame.format

Seems kind of wrong that this wasn't done, although it didn't have any
bad consequences.
af_lavcac3enc: use new AVFrame API
manpage: remove author field

There's already a more detailed "AUTHORS" section.

Closes #647.
vd_lavc: reduce hardware decoder mid-stream reinitializations

Instead of doing it on every seek (libavcodec calls get_format on every
seek), reinitialize the decoder only if the video resolution changes.

Note that this may be relatively naive, since we e.g. (or: in
particular) don't check for profile changes. But it's not worse than the
state before the get_format change, and at least it paints over the
current vaapi breakage (issue #646).
vd_lavc: remove unused field
video: fix FFmpeg or Libav being a special snowflake
vd_lavc: reinit hwdec on profile changes

Needed in theory. I don't know if there are even any real-world files
which change the profile mid-stream.
img_format: AV_PIX_FMT_FLAG_ALPHA is always available

We no more support ancient libavutil versions.