fftools/ffmpeg: stop accessing OutputStream.last_dropped in print_report()

That field is used by the framerate code to track whether any output has
been generated for the last input frame(*). Its use in the last
invocation of print_report() is meant to account for the very last
filtered frame being dropped in the number of dropped frames printed in
the log. However, that is a highly inappropriate place to do so, as it
makes assumptions about vsync logic in completely unrelated code. Move
the increment to encoder flush instead.

(*) the name is misleading, as the input frame has not yet been dropped
    and may still be output in the future

fftools/ffmpeg_enc.c

@@ -749,6 +749,9 @@ static int encode_frame(OutputFile *of, OutputStream *ost, AVFrame *frame)
 
         if (frame->sample_aspect_ratio.num && !ost->frame_aspect_ratio.num)
             enc->sample_aspect_ratio = frame->sample_aspect_ratio;
+    } else if (ost->last_dropped) {
+        ost->nb_frames_drop++;
+        ost->last_dropped = 0;
     }
 
     update_benchmark(NULL);