ffmpeg: add option to control copying of frames prior to start.

Normally we discard things prior to the intended start
for stream copy this is not always possible, and its not done by default
this option allows discarding to be enabled

this is primarely usefull when transcoding a video and stream copying an
audio stream.

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

ffmpeg.c

@@ -1340,6 +1340,10 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
         !ost->copy_initial_nonkeyframes)
         return;
 
+    if (!ost->frame_number && ist->pts < of->start_time &&
+        !ost->copy_prior_start)
+        return;
+
     if (of->recording_time != INT64_MAX &&
         ist->pts >= of->recording_time + of->start_time) {
         close_output_stream(ost);