streamid does not work with newaudio, newvideo, newsubtitle

fixes issue2465.

The problem is that the ffmpeg (the app) -streamid option did not work
with -newaudio/-newvideo/-newsubtitle.

The cause was a conflict between the feature where streamid values were
reset to default for each output filename, and the implementation of
-new*, which requires that the -new* option be specified after the
target filename.

My patch changes the ffmpeg behavior so that user-specified streamid
values apply to all the following output files on the command line
(rather than just the next output filename.)

Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit 20ac9de3df)

ffmpeg.c

@@ -3816,7 +3816,6 @@ static void opt_output_file(const char *filename)
 
     set_context_opts(oc, avformat_opts, AV_OPT_FLAG_ENCODING_PARAM, NULL);
 
-    nb_streamid_map = 0;
     av_freep(&forced_key_frames);
 }