fftools/ffmpeg: Avoid temporary frame

send_frame_to_filters() sends a frame to all the filters that
need said frame; for every filter except the last one this involves
creating a reference to the frame, because
av_buffersrc_add_frame_flags() by default takes ownership of
the supplied references. Yet said function has a flag which
changes its behaviour to create a reference itself.
This commit uses this flag and stops creating the references itself;
this allows to remove the spare AVFrame holding the temporary
references; it also avoids unreferencing said frame.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/ffmpeg.h

@@ -311,7 +311,6 @@ typedef struct InputStream {
     AVCodecContext *dec_ctx;
     const AVCodec *dec;
     AVFrame *decoded_frame;
-    AVFrame *filter_frame; /* a ref of decoded_frame, to be sent to filters */
     AVPacket *pkt;
 
     int64_t       prev_pkt_pts;