lavfi/overlay: remove merge
lavfi/overlay: re-do merge (INCOMPLETE).

Can not build yet, depends on buffersink merge.
lavfi/hue: switch to AVFrame.
lavfi/hue: add an optimized direct path.
lavfi/hue: reindent.
lavfi/colormatrix: switch to AVFrame.
lavfi/histeq: switch to AVFrame.

XXX: can't we have a direct path here?
lavfi/tile: switch to AVFrame.