lavfi: disallow ff_request_frame for filters using activate.

Having two different functions allows to have stricter tests
and detect errors earlier.

libavfilter/avfilter.c

@@ -408,6 +408,7 @@ int ff_request_frame(AVFilterLink *link)
 {
     FF_TPRINTF_START(NULL, request_frame); ff_tlog_link(NULL, link, 1);
 
+    av_assert1(!link->dst->filter->activate);
     if (link->status_out)
         return link->status_out;
     if (link->status_in) {

libavfilter/internal.h

@@ -309,6 +309,9 @@ int ff_poll_frame(AVFilterLink *link);
 /**
  * Request an input frame from the filter at the other end of the link.
  *
+ * This function must not be used by filters using the activate callback,
+ * use ff_link_set_frame_wanted() instead.
+ *
  * The input filter may pass the request on to its inputs, fulfill the
  * request from an internal buffer or any other means specific to its function.
  *