- Gracefully handle the case where not all the streams are requested/wanted

from the client. Simply ignore the unwanted/unasked streams.
- Don't need to pool() for every input character! (the socket is nonblocking,
so the loop is ok).
- Partially resurrect compute_send_delay for avoiding udp flood. Without a
similar patch, udp transmission is seriously unreliable.
(note that we don't link to a specific input reference stream, it's not needed
as the pts values should be coherent anyway. Also, non-monotonic pts
progression is unimportant in the long term).
- rtsp_cmd_pause must reset the time reference
patch by (Giancarlo Formicuccia <ilsensine at inwind dot it>)

Originally committed as revision 2034 to svn://svn.ffmpeg.org/ffmpeg/trunk