1 pass VBR mode bug fix.

The one pass VBR mode selects a Q range based on a
moving average of recent Q values. This calculation
should have been excluding arf overlay frames as these
are usually coded at the highest allowed value. Their
inclusion skews the average and can cause it to drift
upwards even when the clip as a whole is undershooting.

As such it can undermine correct adaptation of the allowed
Q range especially for easy content.

Change-Id: I7d10fe4227262376aa2dc2a7aec0f1fd82bf11f9