Until now, the end_display_time was ignored, making single
packets subtitles (like dvdsub) stay indefinitely.
start_display_time was also ignored, but is it almost always 0.
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>