lavfi/video: remove duplicated function ff_null_start_frame_keep_ref()

Semantic for the function ff_null_start_frame() was changed in
07bad278, and it has now the same behavior of
ff_null_start_frame_keep_ref(), thus it makes no sense to keep both of
them.

libavfilter/vf_bbox.c

@@ -100,7 +100,7 @@ AVFilter avfilter_vf_bbox = {
         { .name             = "default",
           .type             = AVMEDIA_TYPE_VIDEO,
           .get_video_buffer = ff_null_get_video_buffer,
-          .start_frame      = ff_null_start_frame_keep_ref,
+          .start_frame      = ff_null_start_frame,
           .end_frame        = end_frame,
           .min_perms        = AV_PERM_READ, },
         { .name = NULL }

libavfilter/vf_blackdetect.c

@@ -210,7 +210,7 @@ AVFilter avfilter_vf_blackdetect = {
           .config_props     = config_input,
           .draw_slice       = draw_slice,
           .get_video_buffer = ff_null_get_video_buffer,
-          .start_frame      = ff_null_start_frame_keep_ref,
+          .start_frame      = ff_null_start_frame,
           .end_frame        = end_frame, },
         { .name = NULL }
     },

libavfilter/vf_blackframe.c

@@ -130,7 +130,7 @@ AVFilter avfilter_vf_blackframe = {
                                           .type             = AVMEDIA_TYPE_VIDEO,
                                           .draw_slice       = draw_slice,
                                           .get_video_buffer = ff_null_get_video_buffer,
-                                          .start_frame      = ff_null_start_frame_keep_ref,
+                                          .start_frame      = ff_null_start_frame,
                                           .end_frame        = end_frame, },
                                         { .name = NULL}},
 

libavfilter/vf_showinfo.c

@@ -94,7 +94,7 @@ AVFilter avfilter_vf_showinfo = {
     .inputs    = (const AVFilterPad[]) {{ .name = "default",
                                           .type             = AVMEDIA_TYPE_VIDEO,
                                           .get_video_buffer = ff_null_get_video_buffer,
-                                          .start_frame      = ff_null_start_frame_keep_ref,
+                                          .start_frame      = ff_null_start_frame,
                                           .end_frame        = end_frame,
                                           .min_perms        = AV_PERM_READ, },
                                         { .name = NULL}},

libavfilter/video.c

@@ -282,12 +282,6 @@ int ff_start_frame(AVFilterLink *link, AVFilterBufferRef *picref)
     return ret;
 }
 
-int ff_null_start_frame_keep_ref(AVFilterLink *inlink,
-                                                AVFilterBufferRef *picref)
-{
-    return ff_start_frame(inlink->dst->outputs[0], avfilter_ref_buffer(picref, ~0));
-}
-
 int ff_null_end_frame(AVFilterLink *link)
 {
     return ff_end_frame(link->dst->outputs[0]);