QSV: alternate workaround for B-pyramid + keyframe insertion issue.

The original workaround (SVN 6016) got rid of the invalid timestamps,
but did nothing to prevent visual artifacts. Since it doesn't look
like I'll be fixing the underlying cause right away, this will avoid
broken output for our users in the meantime.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6098 b64f7644-9d1e-0410-96f1-a4d463321fa5