Fix garbled previews from BD h.264 sources

Fix garbled previews from BD h.264 sources

Forome reason, frames that are tagged as recovery points in many BD h.264
streams do not result in complete frames when decoded.  Pushing 2 extra
frames through the decoder seems to always fix this.  This patch extends
something I was already doing when generating previews from a BD structure.
This just applies the same logic to ffmpeg streams that have h.264 video.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@3895 b64f7644-9d1e-0410-96f1-a4d463321fa5