lavfi: remove avfilter_null_* from public API on next bump.

Those functions are only useful inside filters. It is better to not
support user filters until the API is more stable.

libavfilter/split.c

@@ -25,6 +25,7 @@
 
 #include "avfilter.h"
 #include "audio.h"
+#include "video.h"
 
 static int split_init(AVFilterContext *ctx, const char *args, void *opaque)
 {
@@ -100,7 +101,7 @@ AVFilter avfilter_vf_split = {
 
     .inputs    = (AVFilterPad[]) {{ .name            = "default",
                                     .type            = AVMEDIA_TYPE_VIDEO,
-                                    .get_video_buffer= avfilter_null_get_video_buffer,
+                                    .get_video_buffer= ff_null_get_video_buffer,
                                     .start_frame     = start_frame,
                                     .draw_slice      = draw_slice,
                                     .end_frame       = end_frame, },