Bump major of libavfilter for reference counted buffer API changes.

Bump major of libavfilter for reference counted buffer API changes.

Applications that have been linked against an older release of Libav and pick up
updated libraries experience segmentation faults because they pick up the new
libavfilter, which assumes AVFrames have been allocated by libavutil and thus
contain new reference-counting related fields. This will break for AVFrames that
have been allocated by old libavcodec.

All scheduled API changes are deferred to the next bump.