Pipe: fill dispose member

Caused leaks in hd-rum-transcoder because capture filter was not calling
vf_free but video_frame::dispose on frame (empty then).

src/video_display/pipe.cpp

@@ -90,7 +90,11 @@ static struct video_frame *display_pipe_getf(void *state)
 {
         struct state_pipe *s = (struct state_pipe *)state;
 
-        return vf_alloc_desc_data(s->desc);
+        struct video_frame *out = vf_alloc_desc_data(s->desc);
+        // explicit dispose is needed because we do not process the frame
+        // by ourselves but it is passed to further processing
+        out->dispose = vf_free;
+        return out;
 }
 
 static int display_pipe_putf(void *state, struct video_frame *frame, int flags)