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>