ffmpeg: better CFR frame duplication selection

This improves the handling of cases where the frame duration is not known

Fixes Ticket 4119
Fixes Ticket 1578

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.h

@@ -388,6 +388,8 @@ typedef struct OutputStream {
     AVCodec *enc;
     int64_t max_frames;
     AVFrame *filtered_frame;
+    AVFrame *last_frame;
+    int last_droped;
 
     /* video only */
     AVRational frame_rate;