demux: allow increasing filepos only

The last demuxed file position (demuxer->filepos) is used to estimate
the total playback percentage in files with possible timestamp resets
(like MPEG-PS). Until know, reading from any stream set this position
freely. This makes the position jump around.

Fix this by allowing icnreasing file position only. Reset it on seeking.
With crazy formats, this still could go wrong, but there's only so much
you can do.