libhb: fix scanning of video that is a single GOP
With this type of stream, scan only successfully decodes 1 frame.
Eliminating the flush after seek allows decoding more frames.
Note that there is a side effect of allowing some junk frames to be decoded,
so we only skip the flush if normal scanning finds only 1 frame.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6396 b64f7644-9d1e-0410-96f1-a4d463321fa5