lavfi: switch to AVFrame.

Deprecate AVFilterBuffer/AVFilterBufferRef and everything related to it
and use AVFrame instead.
mpegvideo_enc: drop support for reusing the input motion vectors.

This misfeature is most likely completely useless and conflicts with
removing the mpegvideo-specific fields from AVFrame. In the improbable
case it is actually useful, it should be reimplemented in a better way.
lavc decoders: work with refcounted frames.
lavc decoders: properly initialize AVFrame.
lavc: allow decoders to override frame parameters.
lavc: limit maximum number of channels to 63

This is the most that can be represented with the current channel layout
system. This limit should be raised/removed when a better system is
implemented.
lavc: make up a fake frame channel layout when there is no real one.

This should ensure that a valid channel layout is always set on a frame,
until a better solution is implemented.
mpegvideo: drop vismv code

It has been broken for over a year without anyone complaining or
noticing, thus proving that nobody ever uses it.
avconv: convert to new refcounted AVFrame API
avplay: switch to new refcounted frames API

Remove now unused cmdutils get_buffer() implementation.
avserver: don't set deprecated options.
lavc: stop setting AVFrame.motion_subsample_log2

It is not used inside lavc anywhere and now it makes no sense to export
it.
AVFrame: deprecate all now unused fields
lavc: postpone the removal of request_channels API.

Its replacement is still not ready.
Add the bumps and APIchanges entries for reference counted buffers changes.
Replace some forgotten instances of PIX_FMT_* with AV_PIX_FMT_*.
Replace remaining includes of audioconvert.h with channel_layout.h
eamad: allocate a dummy reference frame when the real one is missing

CC:libav-stable@libav.org
ffv1: fix calculating slice dimensions for version 2

It got broken in 0f13cd3187192ba0cc2b043430de6e279e7b97c3.

CC:libav-stable@libav.org
FATE: enable multiple slices in the ffv1 vsynth test

This allows us to test the slice threading code.