lavfi/movie: remove merge
lavfi/movie: re-do merge.

Notes:
 - Added a FIXME about the ff_filter_frame() return code unchecked.
 - a lookup on the potential movie->frame memleaks are welcome
 - compilation ok, output untested
 - "buf->pts = av_frame_get_best_effort_timestamp(frame)" <- this wasn't
   kept. It might still be relevant though (check needed when we can
   test it)