ffmpeg: set VCFR when copying timestamps

This fixes unreasonable initial frame repeats
Fixes Ticket3176
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -833,6 +833,9 @@ static void do_video_out(AVFormatContext *s,
             && input_files[ist->file_index]->input_ts_offset == 0) {
             format_video_sync = VSYNC_VSCFR;
         }
+        if (format_video_sync == VSYNC_CFR && copy_ts) {
+            format_video_sync = VSYNC_VSCFR;
+        }
     }
 
     switch (format_video_sync) {