lavfi: add vsink_buffer, and use it in ff* tools

Also add the public interface libavfilter/vsink_buffer.h.

cmdutils.h

@@ -260,19 +260,4 @@ int read_file(const char *filename, char **bufptr, size_t *size);
 FILE *get_preset_file(char *filename, size_t filename_size,
                       const char *preset_name, int is_path, const char *codec_name);
 
-typedef struct {
-    enum PixelFormat pix_fmt;
-} FFSinkContext;
-
-extern AVFilter ffsink;
-
-/**
- * Extract a frame from sink.
- *
- * @return a negative error in case of failure, 1 if one frame has
- * been extracted successfully.
- */
-int get_filtered_video_frame(AVFilterContext *sink, AVFrame *frame,
-                             AVFilterBufferRef **picref, AVRational *pts_tb);
-
 #endif /* FFMPEG_CMDUTILS_H */