avfilter/avfilter: Move frame_pool to FilterLinkInternal

Avoids ugly casts when uninitializing.
(One could actually avoid allocating this separately if one
were willing to expose FFFramePool to those files including
link_internal.h.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/avfilter_internal.h

@@ -33,6 +33,8 @@
 typedef struct FilterLinkInternal {
     AVFilterLink l;
 
+    struct FFFramePool *frame_pool;
+
     /**
      * Queue of frames waiting to be filtered.
      */