libavfilter: default to filter_frame when neither it nor start/slice/end is set.

this is needed for vf_null and a few others.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/video.c

@@ -210,7 +210,7 @@ static int default_start_frame(AVFilterLink *inlink, AVFilterBufferRef *picref)
     if (inlink->dst->nb_outputs)
         outlink = inlink->dst->outputs[0];
 
-    if (outlink && !inlink->dstpad->filter_frame) {
+    if (outlink && (inlink->dstpad->start_frame || inlink->dstpad->end_frame || inlink->dstpad->draw_slice)) {
         AVFilterBufferRef *buf_out;
         outlink->out_buf = ff_get_video_buffer(outlink, AV_PERM_WRITE, outlink->w, outlink->h);
         if (!outlink->out_buf)
@@ -332,8 +332,12 @@ static int default_end_frame(AVFilterLink *inlink)
             int ret = inlink->dstpad->filter_frame(inlink, inlink->cur_buf);
             inlink->cur_buf = NULL;
             return ret;
-        } else {
+        } else if (inlink->dstpad->start_frame || inlink->dstpad->end_frame || inlink->dstpad->draw_slice){
             return ff_end_frame(outlink);
+        } else {
+            int ret = ff_filter_frame(outlink, inlink->cur_buf);
+            inlink->cur_buf = NULL;
+            return ret;
         }
     }
     return 0;
@@ -366,7 +370,7 @@ static int default_draw_slice(AVFilterLink *inlink, int y, int h, int slice_dir)
     if (inlink->dst->nb_outputs)
         outlink = inlink->dst->outputs[0];
 
-    if (outlink && !inlink->dstpad->filter_frame)
+    if (outlink && (inlink->dstpad->start_frame || inlink->dstpad->end_frame || inlink->dstpad->draw_slice))
         return ff_draw_slice(outlink, y, h, slice_dir);
     return 0;
 }