lavfi: make filter_frame non-recursive.

A lot of changes happen at the same time:

- Add a framequeue fifo to AVFilterLink.

- split AVFilterLink.status into status_in and status_out: requires
  changes to the few filters and programs that use it directly
  (f_interleave, split, filtfmts).

- Add a field ready to AVFilterContext, marking when the filter is ready
  and its activation priority.

- Add flags to mark blocked links.

- Change ff_filter_frame() to enqueue the frame.

- Change all filtering functions to update the ready field and the
  blocked flags.

- Update ff_filter_graph_run_once() to use the ready field.

- buffersrc: always push the frame immediately.

libavfilter/tests/filtfmts.c

@@ -25,6 +25,9 @@
 #include "libavutil/pixdesc.h"
 #include "libavutil/samplefmt.h"
 
+#define FF_INTERNAL_FIELDS 1
+#include "libavfilter/framequeue.h"
+
 #include "libavfilter/avfilter.h"
 #include "libavfilter/formats.h"