Use separate fake filters for exporting inputs and outputs from filter graphs.

Use separate fake filters for exporting inputs and outputs from filter graphs.
This makes the code a little less confusing, and is needed for the patch to
improve the link configuration process, to be committed next.

Commited in SoC by Bobby Bingham on 2007-12-24 02:53:56

Originally committed as revision 12716 to svn://svn.ffmpeg.org/ffmpeg/trunk