- the first audio and video packs now contain only a system header and lots
  of padding.
  - no system headers in any packs other than the first ones
  - the two system headers only contain information about "their" stream
  - fixed some header values (muxrate, some flags, ...) so that they have the
  values specified by the standard
  - padding packs are inserted if the mux rate would be below 75 packs per
  second (the rate must not be below or above that value).
  - fixed the SCR of the packs
  - 20 zero bytes are now inserted at the end of each audio pack, after the
  data packet

Originally committed as revision 2804 to svn://svn.ffmpeg.org/ffmpeg/trunk