lavfi: switch ff_default_get_audio_buffer() to av_frame_get_buffer()

This simplifies the code and avoids using libavcodec-specific
avcodec_fill_audio_frame().
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  lavfi: switch ff_default_get_audio_buffer() to av_frame_get_buffer()

Conflicts:
	libavfilter/audio.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
sonicenc: set supported samples formats array

This ensures that only supported formats are input

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>