af_lavrresample: add new resampling filter to replace the old ones

Remove `af_resample` and `af_lavcresample`. The former is a mess while the
latter uses an API that was long deprecated in libavcodec and is now removed.

`af_lavrresample` rougly has the same features and structure of
`af_lavcresample`.
af_lavrresample: fall back to libswresample

This should make it work with ffmpeg with standard compilation flags.
(ffmpeg ships libavresample, but does not enable its compilation and
installation by default.)

Since the APIs are extremely similar, a minor #define hell is enough to
handle this.