Preventing screensaver when playing movie?

[Erik]

Suppose that we have the following situation. A user has told the system 
to "turn off the display after N minutes of inactivity". Then he starts 
the media player, opens a file and presses a button called play. The 
media player interprets this as the user saying "play this movie, but do 
not turn off the display while playing it, because i will watch it even 
though I do not interact with the system".

So the media player has to do something like this. Each time the 
application updates the display with a new animation frame, it marks 
that update with a special flag, indicating that this is the kind of 
thing that the user may be watching even though he has not interacted 
with the system
for a while. System then resets the screensaver countdown. However it 
does not reset the session locking countdown, because only real input 
may do that, for security reasons. So the user may have to give his 
password when he wants to interact with the system after playing the movie.

How can the media player achieve this with xorg?