lavfi/pan: switch to AVFrame.
lavfi/silencedetect: switch to AVFrame.
lavfi/showspectrum: switch to AVFrame.
lavfi/ebur128: switch to AVFrame.
lavfi/edgedetect: switch to AVFrame.
lavfi/geq: switch to AVFrame.
lavfi/pp: switch to AVFrame.

Note: qp not honored anymore...
lavfi/overlay: remove merge
lavfi/overlay: re-do merge (INCOMPLETE).

Can not build yet, depends on buffersink merge.