lavfi: do not call ff_filter_frame() with activate.

avfilter_graph_request_oldest() does work that should be done by
either the filter or the application.

The principle of this function, calling ff_request_frame() from
outside the filter was always shaky. This version is less elegant
since it requires making special cases for each filter, but it
is more robust since it no longer calls ff_request_frame()
directly without notifying the filter.

Eventually, avfilter_graph_request_oldest() will be deprecated
for a function to just run the graph.

libavfilter/avfiltergraph.c

@@ -36,6 +36,7 @@
 #include "framequeue.h"
 
 #include "avfilter.h"
+#include "buffersink.h"
 #include "formats.h"
 #include "internal.h"
 #include "thread.h"
@@ -1389,6 +1390,11 @@ int avfilter_graph_request_oldest(AVFilterGraph *graph)
 
     while (graph->sink_links_count) {
         oldest = graph->sink_links[0];
+        if (oldest->dst->filter->activate) {
+            /* For now, buffersink is the only filter implementing activate. */
+            return av_buffersink_get_frame_flags(oldest->dst, NULL,
+                                                 AV_BUFFERSINK_FLAG_PEEK);
+        }
         r = ff_request_frame(oldest);
         if (r != AVERROR_EOF)
             break;
@@ -1403,6 +1409,7 @@ int avfilter_graph_request_oldest(AVFilterGraph *graph)
     }
     if (!graph->sink_links_count)
         return AVERROR_EOF;
+    av_assert1(!oldest->dst->filter->activate);
     av_assert1(oldest->age_index >= 0);
     frame_count = oldest->frame_count_out;
     while (frame_count == oldest->frame_count_out) {